Data transfer speed from hard drives, and therefore your System and application performance, is governed primarily by two major things, RPM and Interface type. This article will attempt to clarify the mysteries, and help you decide what kind of drive(s) to use in your Mac. There's a lot of information to absorb in one sitting, but we'll try to make it as simple as we can.
The first aspect of a drive's performance is its physical rotation speed, or how fast it can spin its media platen (plates or platters), measured in RPM (Revolutions Per Minute). The faster a drive spins its disks, the more data it can read in a given time.
Most older drives ranged from about 2400 RPM to 3600 RPM, while today's low-end drives start at 5400 RPM, and high-performance drives are at 7200 RPM, 10,000 RPM, and now also 15,000RPM. Faster spinning drives are always on the horizon, but you will always pay a premium for the fastest available.
Please note: Portable drives for laptop computers are currently at a considerably slower RPM than desktop drives; but because of increased bus speeds, the latest Powerbook and iBook models offer exceptional performance. Drives in early model Powerbooks are the cause of serious bottlenecks, and drastic performance increases may be had for no more than the cost of a new internal drive. I highly recommend the IBM Travelstar 5400RPM series.
Because of the nature of how a hard drive is engineered, some parts of the drive media can be read from and written to faster than others. The outside of a drive is faster than the inside, because a single line around the outside of the drive media contains more physical blocks than a line around the inside, thus, as the drive spins at a fixed speed, the drive head can read or write more blocks in a single revolution around the outside than on the inside.
If you've ever seen an old vinyl LP record, then you can easily compare the two. An LP always spins at a fixed speed (e.g., 33 RPM), and the stylus (needle) can read more linear information per revolution on the outside than the inside. The major difference between an LP and a hard drive, of course, is that the hard drive can be easily written to (record) over and over, instead of just being read from (playback).
In short, the total length of a line (circle) around the outside of a disk is longer than one on the inside, and so may contain more information. The read-write head of a drive travels (relevant to the motion of the disk beneath) at the same speed over each circle (line), so with each pass (revolution), it can read more on the longer (outside) line. Make sense?
If not, don't worry; just know that it is true, and see Partitioning Your Hard Drives for more information on how to exploit this "feature".
There are many types of interfaces used in Macintosh computers to communicate (transfer data) with hard drives, and recently divided into two major types, SCSI and ATA. Each has its advantages and disadvantages.
I can already hear you asking, "What about USB? FireWire? Parallel? Ethernet? FibreChannel? Can't I also buy an external USB (or other) drive for my iMac?".
While it's true that there are now more ways than ever to attach a drive or peripheral to your Mac, when it comes to basic hard drives, they are all still either SCSI or ATA at the core, adapted in an external container to use one of the named interfaces. Native FireWire drives are overdue, and no realistic target date for release has been announced as of this writing; and FibreChannel is still extremely expensive for the performance and size delivered. So, for now, let's look at the two major kinds, and discuss the best way to hook them up later.
Quick Chart
| Mac Model | OEM ATA Type | OEM SCSI Type | Optional OEM PCI SCSI Type |
| Pre-'98 601/604 PPC and Clones | EIDE (if present) | SCSI-1 and/or SCSI-2 | Ultra SCSI |
| '98 G3 (Beige; Gossamer, Artemis) | EIDE-16 | SCSI-1 | UltraWide SCSI |
| '99 G3 (Blue & White; Yosemite) | ATA-33 | none | SCSI-2 / Ultra2 SCSI |
| '99 G4 PCI (Graphite; Yikes!) | ATA-33 | none | SCSI-2 / Ultra2 SCSI |
| '99 / '00 G4 AGP (Graphite; Sawtooth) | ATA-66 | none | SCSI-2 / Ultra2 SCSI |
| '98 iMac (RevA-B; Bondi) | EIDE-16 | none | none |
| '99 iMac (RevC; Fruit Flavors) | ATA-33 | none | none |
| '99 / '00 iMac (RevD; DV) | ATA-66 | none | none |
| '00 / '01 G4 and Cube | ATA-66 | none | SCSI-2 / Ultra3 LVD SCSI |
| '00 / '01 iMac | ATA-66 | none | none |
| '00 / '01 Powerbook and iBook | ATA-66 | none | none |
SCSI, pronounced "Skuhzee", stands for Small Computer Systems Interface, and it came standard on most PowerPCs and older Macs since 1987(?). The 1999 PowerMac G3 Desktop models (blue & white) are the first series in years to not employ built-in SCSI ports, instead opting for ATA as stock, and allowing the interface of SCSI devices via PCI expansion cards; the new tradition continues throughout the G4 lineup.
SCSI can also host (interface with) many types of peripheral devices, such as scanners, printers, removable-media drives (e.g., Zip, Jaz, Orb, etc.), and has an added advantage of allowing drives to be installed both inside (internal) and outside (external) of the computer. Further, each SCSI interface controller, also called a Bus, can host at least seven devices in addition to the Macintosh itself.
These host-points are called Addresses or ID Numbers, and devices use the numbers 0-6, while the Macintosh CPU resides at number 7. By installing a second SCSI controller, usually in a PCI expansion slot inside the computer, you can add more addresses, and thus host more devices on the new Bus (requires SCSI Manager 4.3, not available on all older Macs). By adding better or more than one controller, you can add at least fourteen SCSI devices, and as many as twenty-one or more; providing you have available power, cooling and interface slots.
OK, now it's time to take a deep breath. . .
SCSI is constantly being re-engineered to be faster and faster, using the same basic interface language, which is very important for maintaining backwards-compatibility with older devices. A first-generation SCSI drive made six (or more) years ago can still coexist with a modern, sixth-generation drive made today, and can even do so on the same Bus without penalty, when a special controller is used.
This chart shows the major SCSI types available for Macintosh computers. There are a bunch of confusing names out there, different ones from different drive-makers, but this covers the bulk (we hope!).
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| Narrow | SCSI-1 | 5MB/sec. max. | 2-4MB/sec. avg. | 50-pin ribbon | 25 pin D-Type (DB-25) |
| Fast | SCSI-2 | 10MB/sec. max. | 7-9MB/sec. avg. | 50-pin ribbon | 25 pin D-Type (DB-25) |
| Fast&Wide | SCSI-3 | 20MB/sec. max. | 15-17MB/sec. avg. | 50-pin ribbon | 50 pin D-Type (HD-50) |
| UltraNarrow | UltraSCSI | 20MB/sec. max. | 16-18MB/sec. avg. | 50 and 68-pin ribbon | HD-Type (HD-xx) |
| UltraWide | SCSI-4 (rare) | 40MB/sec. max. | 22-33MB/sec. avg. | 68-pin ribbon | 50-pin HD-Type (HD-50) 68-pin HD-Type (HD-68) |
| Ultra2Wide | U2 LVD | 80MB/sec. max. | 56-70MB/sec. avg. | 68-pin ribbon | 68-pin HD-Type (HD-68) |
| Ultra3Wide | Ultra160 | 160MB/sec. max. | 74- 157MB/sec | 68-pin ribbon | 68-pin HD-Type (HD-68) |
| Ultra4 | Ultra320 | 320MB/sec. max. | ? (due Fall 2001) | 80-pin ribbon | ? |
As you can see, each generation tends to double the maximum data transfer rate of its sibling, measured in Megabytes per second. But, as noted earlier, the real-world results are typically much slower, mostly because of the RPM and the drives ability to read data faster from the outer portions of the platen, compared to the inside. But, the higher the RPM, the closer to the maximum data-transfer rate you'll get. One may also combine, or 'Stripe' drives into a RAID array in order to further saturate the bus in order to reach the highest rates noted above. (See: RAID: It Kills Bottlenecks Dead for more information)
ATA is a general term also known as Atapi, IDE, EIDE, and sometimes UDMA. This is the same kind of drive found in most Windows/Intel computers (Wintel), and the ATA controller interface ports are present in all G3, iMac, PowerBook and iBook computers. It is also found in certain older xx00 and Performa series Macs. It is also an interface that is and was the focus of much controversy when it was brought over to the Mac world.
While extremely affordable and readily available, its performance typically suffered greatly when compared to like-generation SCSI, which is what most older Macs always used. This was always one of the chief reasons a Mac was faster than a Wintel, but it was also a reason Macs would cost considerably more than a Wintel.
You may have noticed that Macs are a lot more affordable than ever these days, and part of this is because of ATA hard drives shipping as stock on low-end models, and most every CD/DVD and Zip drive shipped stock is ATA/EIDE (Atapi).
For the most part, only hard drives, CD/DVD-ROM drives, and removable media drives can be attached to ATA/EIDE interfaces. There are no peripheral devices like scanners or printers for ATA/EIDE, in part because it does not have the ability to safely have an external Bus like SCSI (unless used with a USB-ATA, SCSI-ATA or FireWire-ATA adapter). ATA also has a two-device per bus limit, usually described as Master and Slave.
(Note: early revisions of certain Mac models can only host one ATA device per bus. Check your user manual to find out how many yours can support; Revision A '98 G3 owners may purchase a Rev B or C ROM card to overcome this limitation)
Interestingly enough, the new Airport wireless interface card appears to use an EIDE Bus to interface with the Mac. We're still looking for some creative third-party providers to develop additional technology for this port; they've already brought AirPort to your older Desktop PowerPC, G3, Powerbook or iMac with relative ease.
ATA, like SCSI, also has faster versions of speeds based on how the data is transferred. And, like SCSI, ATA/EIDE drives are constantly evolving, creating faster and faster maximum transfer rates. However, if you own an older Mac, and unless you buy a special high-performance PCI-based controller-card for fast EIDE devices (like UATA and UDMA), it's all pretty much a moot point, because the built-in ATA controller on older Macs ( in the 1995-1997 era) is the slowest you can get, topping out at a theoretical maximum of 8 Megabytes per second*. The good news is that you can now buy one of these controllers for well under $100.
Worse, until very recently, the stock drives that Apple ships with consumer PowerMacs and iMacs are the least-expensive available, and thus the slowest speed available (5400 RPM), so an older Mac's stock ATA data rate is something like 2-4MB/sec. avg., when the drives are maintained for optimum performance (see: Partitioning Your Hard Drives for more information). This is comparable to Narrow SCSI-1 speeds, such as that found in Zip drives and older Macs, and most Mac-external SCSI speeds.
While older Macs were effectively crippled by the use of EIDE/ATA hard drives as the primary devices, this is no longer true since the introduction of faster ATA drives and bus speeds; new PowerMacs ship with much better versions, so even 5400RPM drives return very high speeds (in excess of 18MB/sec). If you need to increase your Mac's performance, and can't afford a move into a fast SCSI option that allows for RAID striping, like UltraWide or Ultra2Wide drives and the needed controller, you can finally reasonably compromise by investing in a 7200 RPM ATA drive, which will do a pretty good job of saturating the internal built-in ATA bus, returning results as high as 6-7MB/sec on older Macs, and as much as 34MB/sec on newer Macs.
Even more importantly, most any PCI Mac (like Performas and 7200-9600, G3/G4 models) can add a PCI-ATA controller card for as little as US$99.00, which will allow you to add the larger, cheaper, and in the case of SCSI-1, SCSI-2 and SCSI-3 based Macs, faster hard drives; giving your Mac new life, and, as with most any drive, you can always take it with you to your next Mac.
Further, a nifty side effect of using a PCI-ATA controller is that they are made to fool the Mac into thinking you're adding SCSI devices. Why is this good? Because it allows you to use existing RAID software to strip them across channels for awesome performance that even rivals Ultra2 SCSI speeds for a fraction of the cost per megabyte.
ATA is still not Ultra3Wide SCSI by a long stretch, but it might be as little as 30% of the cost for like-RPM, one-third-size UW drives and controller. If you already have a high-speed SCSI-type controller, or are a professional whose income depends on maximum performance on a daily or hourly basis, don't settle for ATA drives when you need all-out performance. Only buy ATA in that situation for cheap storage. However, the average consumer and prosumer, and even many classes of pro audio and video users can easily use fast ATA, especially when combined with RAID 0 for incredible performance. Just note that some audio and video applications do not always play well with PCI-ATA controllers, and that, to date, they are not compatible with OS X (all makers have announced upcoming firmware updates to resolve this, however); you may need a G3 or G4 Mac with native ATA to take advantage of these drives, or buy one of the more expensive controllers from people like VST and Promax, who are in the business of audio and video HP I/O.
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| EIDE | (?) | 8MB/sec. max.* | 2-4MB/sec. avg. | 40-pin ribbon | none |
| EIDE-16 | ATA | 16MB/sec. max. | 4-15MB/sec. avg. | 40-pin ribbon | none |
| ATA-33 | UDMA-33 | 33MB/sec. max. | 12-31MB/sec. avg. | 40-pin ribbon/80conductor (opt.) | none |
| ATA-66 | UDMA-66 | 66MB/sec. max. | 16-34MB/sec. avg. | 40-pin/80 conductor ribbon | none |
| ATA-1001 | UDMA-100 | 100MB/sec. max | 16-34MB/sec. avg. | 40-pin-80 conductor ribbon | none |
| Serial ATA | ? | 100MB/sec. plus | ? (due Fall 2001) | Serial ATA cable | ? |
* Information on this data rate may be incorrect; white paper specifications were unavailable at the time of publication.
** Sustained and Burst Read and Write speeds are dramatically affected by processor speed, drive quality, RPM, driver, controller, and condition of Directory. Use a benchmark utility to determine your own performance.
1ATA-100 drives are backwards-compatible with earlier EIDE and ATA specs; but ATA controllers are not yet available on the Mac.
Remember, in addition to the language of the interface, which dictates a maximum transfer rate, the RPM of the drive is a big factor in how fast you can move data on a particular Bus-Type. Also, the number of devices on a single Bus, and whether or not each is being accessed by your Macintosh processor will affect the speed of data reading and writing at. You must combine all these factors when you plan out how fast your drives might be accessed at any given time.
Think of the Mac like a city, and each drive or device is a place to go or to get things from. The RAM or System memory is among those places. Each place is connected by ribbons or cables, much like roads, which, when combined with controllers, much like traffic lights, are designed to let traffic move at a given rate.
The Mac processor, in cooperation with the Disk Cache Manager, is a combination of a Master traffic cop and a delivery manager, instructing each device to move data to and from each other. The faster the processor, the more instructions it can give to the drives and the data moving in between. Faster drives can provide more traffic on the roads, faster interface languages can pack more cargo (data) in each vehicle (packet), and so you have to make sure your most important stuff is given higher priority than others.
You can help the cop (processor), the delivery manager (Disk Cache) and the traffic lights (Bus controllers) determine what is most important by storing your most critical data at more important addresses. Just like certain cities give better services to certain parts of town, your SCSI and ATA devices can also be given better addresses.
With ATA, Bus 0 is given higher priority than Bus 1, and address 0 is given higher priority than address 1. That said, when two drives are asking for access at the same time on the same ATA channel, each drive has to alternate access with the other. This is a fast way to cripple your HDD performance when copying data from one drive to the other, or accessing both drives at the same time. Whenever possible, always place your primary-use HDDs on different ATA channels; buy a PCI-ATA controller card if you must to accomplish this.
With SCSI, an address of 0 is given higher priority than a higher number, with priority ascending up the address line (e.g., 2 is faster than 3, 3 faster than 4, etc.). The Bus speed is dependant on type, with each controller being able to talk to the processor at a fairly equal priority, if not actual speed.
Further, you can speed up the shipping and receiving process itself by giving your most critical data the best place to live within each address, i.e., stored on the fastest-accessible portion of the drive (see: Partitioning Your Hard Drives). And, unlike ATA, SCSI does not suffer from drive access issues; each drive may push as much data as it can at the same time as the others, limited only by the max bandwidth of the Bus itself. This, among other reasons, such as faster RPM, larger disk-cache memory and more-reliable construction, is why SCSI remains the better choice for high-end and mission-critical work.
Again, other tricks and tools can be employed to increase read-write traffic performance, such as with RAID arrays.
Each SCSI and ATA device uses power, and your Macintosh power supply may not be rated to safely host a large number of devices. You will need to consult your manual and specifications to determine the maximum amount of power your Macintosh can provide. Don't try to power more devices than your power supply can handle, or else your drives might be damaged, your data lost in the process, and your power supply might overload and burn out. If you need to power more devices than you have stock power, you will need to explore the use of additional or better power supplies, or use an external expansion chassis or drive enclosure with its own power supply.
When working with both ATA and SCSI devices, one must pay very close and careful attention to the total length of cables employed to connect all the devices. Consult your manual and specifications sheets for the exact length of a safe configuration.
You may think of it like a very long road, and each device needs enough fuel to reach its destination. A device can only supply itself with a finite amount of energy, and if you make the combined roads too long, data moving on any road may run out of power, and fail to reach its destination, or simply take too long to get there.
In case you can't tell, this is a bad thing, and potentially a very bad thing. One common mistake we see these days are people using too long a cable on the ATA-66 Bus; specifications require a cable no longer than 18" in length, but we find that even exceeding 12" on the PowerMac internal ATA bus is problematic. Similarly, we continually find external SCSI chains in lengths much longer than required, and a failure to also account for the length of internal cables on the same controller. Regardless of Bus-type, the shorter the cable, the better.
You must also pay attention to the quality of the cable. Cheap or damaged cables, like roads of the same, can cause traffic jams, wrecks, spills, and even permanent damage. Keep your kids, dogs and cats away from your external cables, and if they appear damaged, have them professionally inspected and tested, or simply replace them with high-quality new ones. External SCSI cables are especially vulnerable to damage, EMF and other types of signal noise, and are frequently the source of considerable instability and short-live hardware. Do not skimp in this venue, and don't be afraid to either invest in good SCSI diagnostic tools and high-end termination devices, or to employ a consultant to inspect and verify your SCSI chain for problems.
Many Macintosh cases place severe limitations on the available amount of internal drive bays, and also may limit the size of those bays. Quite often, the space available is a pretty good indication of how much power your Mac can supply, and therefore how many internal drives you should try to support with a stock power supply unit. Again, check out your specifications carefully, and do not exceed the maximum rating.
External drive cases supply their own power and cooling, so you need only provide and available address on the appropriate Bus, and a quality cable that does not exceed the maximum length recommended by the controller provider or manufacturer.
All that said, a creative soul can always find a way to stuff more into a box than it was intended to hold, so as long as you're sure you've got adequate power and available addresses on a bus, you'll find that the most performance and the least-expensive costs can be had by utilizing internal, rather than external drives. Just be sure to provide adequate cooling, usually in the form of additional fans inside the case directed across the new drives.
That, my Friends, is the hardest question of all. Without a personal consultation, it's hard to tell you for sure. So we have created a little chart to try to help you out. The drive type and RPM listed is the absolute minimum we would advise for the purpose. As always, faster is better, bigger is also often a consideration. If you are seeking the ultimate in read-write performance, as well as additional data safety, you may want to explore RAID arrays. You will need to balance things out, according to your needs and budget.
The chart below is divided into three user-types; Professional, SOHO, and Consumer. One could easily define fifty subcategories, but I'm just not interested in doing so! Consider the Professional category somewhere between bleeding-edge and average for high-end, multi-Mac shops, the SOHO the very rough middle of one-horse-stables, and the Consumer division the bottom line for infrequent, casual, non-paid computer use. The drive type indicated in each category is the minimum specification we advise; always go faster if you can afford it.
| Application | Example Software | Professional | SOHO | Consumer |
| Data Base | Filemaker, ClarisWorks | UltraSCSI or ATA-66 7200 RPM |
SCSI-2 or ATA-33 5400RPM |
SCSI-1 or EIDE 5400RPM |
| Word Processing | MS Word, ClarisWorks, WordPerfect | SCSI-1 or ATA-33 5400RPM |
SCSI-1 or ATA-33 5400RPM |
SCSI-1 or EIDE 5400RPM |
| Spread Sheets | MS Excel, ClarisWorks, Lotus | UltraSCSI or ATA-66 7200RPM |
UltraSCSI or ATA-33 5400RPM |
SCSI-1 or EIDE 5400RPM |
| Audio Capture/Editing | Digital Perfomer, RayGun, Coaster | Ultra2Wide 10k RAID Arrays |
Ultra2Wide or ATA-66 7200RPM |
SCSI-2 or ATA-33 7200RPM |
| Video Capture/Editing | Premier, Final Cut, Media XL, QuickTime | Ultra160 10k RAID Arrays |
Ultra2Wide or ATA-66 7200RPM RAID Arrays |
UW SCSI or ATA-33 7200RPM |
| Photo Capture/Editing | Photoshop, PhotoDeluxe, Graphic Converter | Ultra2Wide 10kRPM |
UltraWide or ATA-66 7200RPM |
SCSI-2 or ATA-33 5400RPM |
| 3D Creation/editing | Maya, AutoCAD, DenebaCAD, | Ultra160 10kRPM |
UltraWide or ATA-66 7200RPM |
SCSI-3 or ATA-33 7200RPM |
| Animation | Flash, Director, SVG | Ultra2Wide or ATA-66 7200RPM |
UltraWide or ATA-66 7200RPM |
SCSI-2 or ATA-33 7200RPM |
| Publishing | Quark, InDesign, Acrobat | Ultra2Wide 7200RPM |
UltraWide or ATA-66 7200RPM |
SCSI-2 or ATA-33 7200RPM |
| Illustration/Drawing | Illustrator, Freehand, Canvas, ClarisWorks | UltraWide 10kRPM |
ATA-66 7200RPM | SCSI-1 or EIDE 5400RPM |
| Internet Development | Adobe GoLive, CyberStudio, DreamWeaver | UltraWide 7200RPM |
UltraWide or ATA-33 7200RPM |
SCSI-1 or ATA 5400 |
| Internet Surfing | Netscape, Explorer, iCab | ATA-66 7200RPM |
ATA-33 7200 |
SCSI-1 or EIDE 5400RPM |
| Power Games | Myth, Myst, Quake, Doom | n/a | n/a | UW or ATA-33 7200RPM |
| Simple Games | Solitaire, Trivia, ToyStory | n/a | n/a | SCSI-1 or EIDE 5400RPM |
| Windows 95/98/NT | VirtualPC, SoftWindows | ATA-66 7200RPM |
ATA-66 7200RPM |
SCSI-2 or ATA-33 7200RPM |
You can consult your user manuals and product specifications sheets to determine what interface and drives came with your Mac, and you can also use Apple System Profiler v2.1.2 or newer to assist you in determining what kinds of devices are installed and where.
What and where, however, are a far cry from how fast. In addition to opening the case on your Mac, reading the drive labels, confirming the cables are connected to the proper ports, and visiting the manufacturers website to read both the actual drive type and speed, as well the inflated performance specifications sheets, you can use utilities like MacBench, and other disk-oriented utilities like FWB Hard Disk Toolkit, which may contain some simplistic or comprehensive disk bench-testing features to determine how well your drives are performing.
By using some common sense, and consulting the articles found here, you can attempt to increase those bench marks to the most you can get without spending more money.
Hopefully, this article has helped you to understand what kinds of drives are available, and how to pick the right drive for your needs. The only other thing I might add is that you can generally take a drive with you to a new Macintosh or a Wintel, and will even be useable several years from now, so you should try to think of the future when you buy. E.G., if you see a great deal on an UltraWide 7200 Seagate Barracuda, but you only have a Performa now, remember that the drive will work on a Narrow SCSI-1 Bus, it just won't live up to its full potential until you can afford an UW controller card, or you move up to a newer Mac with one already there. This is especially true of the new, fast ATA-66/100 drives. As of today's writing, you can get a super-fast (in excess of 34MB/sec.) 7200RPM ATA-66/100 drive with 45GB(!) of storage for under $140(!). The largest drive available as of this writing is 80GB, and costs well under $800. Incredible.
Article last revised on: 1 March 2001
Article Created On: 10 August 1998
Author: Frederico
Editor: WebClub '99 / '00 / '01
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