Low End Mac's Online Tech Journal

Hard Drives

Dan Knight - 1999.08.27

In the beginning, personal computers used cassette tapes to store programs and data files. The Apple II, Commodore PET, and TRS-80 had very little memory, so it didn't take too long to load or save files.

The big breakthrough in that era was Steve Wozniak's incredibly inexpensive floppy drive controller for the Apple II, which made floppy drives much more accessible. And the 143 KB capacity combined with instant access to any sector was simply amazing.

Hard drives started to come into their own after Visicalc, as personal computers were starting to find a home in the office. At $4,000 for 5 MB, they were far from a mass market item at first.

Twenty years later we take multi-gigabyte hard drives for granted and can often find last month's model on sale for under $200 (and sometimes under $100).

The basic technology hasn't changed much. Precisely machined platters spin at several thousand RPM. Tiny heads barely avoid touching the surface, reading or writing data at millions of bits per second.

What has changed is performance. Twenty years ago a hard drive might spin at 3000 RPM, hold 5 or 10 MB of information, and fill a full-height 5-1/4" drive enclosure.

Today's hard drives spin at up to 10,000 RPM, store up to 50 GB, and come small enough to fit in a laptop. Most drives are 3.5" designs only about one inch thick.


Back in the era of the IBM PC-XT, the MFM drive was the norm. Then came RLL, which managed about 50% more data on the same drive. Soon came the AT standard, upon which is based the IDE standard seen in most of today's computers.

Apple took a different route, adopting SCSI in 1986. SCSI drives had to be more intelligent than AT drives, which required a bit more computer overhead. Further, SCSI drives could be chained up to 7 per port. AT drives were maybe two per controller.

Of course, with IBM and MS-DOS setting the standard, over time the Wintel sector grew to about 90% of the market. Sheer numbers dictate that all else being equal, market size would make IDE drives less costly.

But all things are not equal. IDE drives had less intelligence, so they were less costly to build than SCSI drives - even when the drives shared almost every component.

In 1994, Apple began using IDE drives in consumer computers. The Quadra 630 and PowerBook 150 were the first IDE Macs. Today IDE is the norm, although you can get a SCSI version of the Blue Power Mac G3.


There has been a back-and-forth war between the two standards. Used to be we saw IDE as slow compared with SCSI. It wasn't necessarily true, but that was the spin. (Comparison of a Quadra 630's IDE drive and a Quadra 650's SCSI drive under Speedometer showed virtually identical performance.)

SCSI has several advantages, but speed isn't one of them. Both IDE and SCSI support data rates far beyond what today's drives can produce.

For the record, SCSI does allow more devices and does require less computer overhead. It is a superior technology, but most of the time IDE is good enough.

Faster, Faster

The typical SCSI connection today is Ultra SCSI, which allows data throughput up to 40 MB/sec. Ultra DMA/33 IDE falls 25% behind at 33 MB/sec. (FireWire supports up to 50 MB/sec.)

Finding a drive that can move data that quickly is another story.

There are three ways a hard drive can speed up throughput:

  1. Spin faster.
  2. Put more data in each track.
  3. Use a big cache.

Spin Faster

Although I haven't used 10,000 RPM drives, I've heard they are incredibly responsive. All things being equal, spinning a drive faster moves the data past the head more quickly. This results in a linear increase in speed.

That is, if you could take a 5400 RPM drive capable of 5.4 MB/sec. and spin it at 7200 RPM, throughput would increase to 7.2 MB/sec. At 10,000 RPM, that would jump to 10 MB/sec.

Increase Data Density

Instead of spinning faster, which also makes the drive run hotter, you can also write the data bits closer together. Take a drive with 17 sectors per track and increase it to 23 sectors per track for an immediate 35% performance boost.

Cache as Cache Can

The third way to improve throughput is a bit of a cheat. Put some memory on the hard drive and let it store data, temporarily buffering it. If the computer needs the data in the buffer, the drive can provide that data more quickly from memory than by reading it from the drive platters.

Sum of the Parts

Today's hard drives are the sum of several parts. While laptop drives are slower to conserve power, most desktop computers have drives spinning at 5400 to 10,000 RPM. Data density varies from drive to drive - and often over the platter within a drive. And buffers have grown as large as 2 MB in some cases.

You'd think it would be easy to find the fastest drive: get the one that spins the fastest.

But that's not the way it works.

The current APS catalog lists dozens of drives on page 10. The fastest write isn't on a 10,000 RPM drive. The APS I 18 GB LVD scores 15.9 MB/sec. while spinning at 7200 RPM. The fastest 10K drive is the APS I LVD 18 GB LVD 10K, which runs a slightly slower 15.7 MB/sec.

Their both wicked fast IBM mechanisms, but somehow the slower spinning drive edges out the vastly more expensive 10K drive.

Regardless, look at those maximum sustained throughput rates. Ultra SCSI and Ultra ATA can handle a lot more throughput, but these drives (some of today's fastest) can't fill that bandwidth.

It does matter which drive you buy. An economy 5400 RPM drive just doesn't offer the responsiveness of a 7200 RPM drive, but at 10,000 RPM you could be paying for performance you'll never see in the real world.

The same goes for your drive interface. Even the older Wide SCSI standard was faster than any of these drives, providing for up to 20 MB/sec. throughput.

And many drives are still rated at below 10 MB/sec., which is the old 8-bit SCSI-2 standard.


Today you can add a ProMAX ATA card and one or more IDE drives to any PCI Power Mac. Or you can drop in a SCSI card and one or more SCSI drives.

Both interfaces offer more bandwidth than today's drives, so for best performance, focus on the drive specifications - especially sustained read and write speeds. Don't be fooled by 10K drives, since they may not offer better throughput than 7200 RPM drives, and they will always cost more.

The only other considerations are price and reliability, and not necessarily in that order.

The two reasons to choose SCSI over IDE are that you already have SCSI devices or that you want to chain several external devices to the computer.

The main reason to choose IDE is bang for the buck. We recently added 16.8 GB hard drives to a couple Blue G3s at work. Even including the cost of the ProMAX controller, they were only 60% as expensive as SCSI drives.

Regardless, you have never been able to buy more megabytes per dollar than you can today, whether SCSI or IDE.

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