To make the best decision in purchasing a hard disk for your system or to understand what distinguishes one brand of hard disk from another, you must consider many features. This section examines some of the issues you should consider when you evaluate drives:

• Capacity

• Performance

• Reliability

• Cost

Capacity

As stated earlier, a corollary of Parkinson's famous "law" can be applied to hard drives: "Data expands so as to fill the space available for its storage." This of course means that no matter how big a drive you get, you will find a way to fill it.

If you've exhausted the space on your current hard disk, you might be wondering, "How much storage space is enough?" Because you are more likely to run out of space than have too much, you should aim high and get the largest drive that will fit within your budget.

Modern systems are used to store many space-hungry file types, including digital photos, music, video, newer operating systems, applications, and games.

As an example, according to hard drive manufacturer Western Digital, storing 600 high-res photos (500KB each), 12 hours of digital music, 5 games, 20 applications, and just 90 minutes of digital video requires an estimated 43GB of space, exceeding the capacity of the typical hard drive installed in many low-end systems sold at retail.

Running out of space causes numerous problems in a modern system, mainly because Windows, as well as many newer applications, uses a large amount of drive space for temporary files and virtual memory. When Windows runs out of room, system instability, crashes, and data loss are inevitable.

Don't be fooled by the interface transfer rate hype, especially around ATA-133. As you can see from the table, a far more important gauge of a drive's performance is the average media transfer rate, which is significantly lower than the interface rate of 133MBps.

The media transfer rate represents the average speed at which the drive can actually read or write data. By comparison, the interface transfer rate merely indicates how fast data can move between the motherboard and the buffer on the drive.

The rotational speed of the drive has the biggest effect on the drive's true transfer speed; in general, drives that spin at 10,000rpm transfer data faster than 7,200rpm drives, and 7,200rpm drives transfer data faster than those that spin at 5,400rpm.

I'm sure many people will trade in an ATA-66 or ATA-100 drive for an ATA-133 model (plus the requisite adapter card or motherboard upgrade) only to find that the ATA-133 drive actually runs about the same speed as or even slower than their previous drive! To prevent this mistake from happening to you, be sure to check the true media transfer rates of any drives you are comparing.

Capacity Limitations

How big a hard drive you can use depends somewhat on the interface you choose. Although the ATA interface is by far the most popular interface for hard drives, SCSI interface drives are also available. Each has different limitations, but those of ATA have always been lower than those of SCSI.

When ATA was first created in 1986, it had a maximum capacity limitation of 137GB (65,536x16x255 sectors). BIOS issues further limited capacity to 8.4GB in systems earlier than 1998, and 528MB in systems earlier than 1994. Even after the BIOS problems were resolved, however, the 137GB limit of ATA remained.

Fortunately, this was broken in the ATA-6 specification drafted in 2001. ATA-6 augments the addressing scheme used by ATA to allow drive capacity to grow to 144PB (petabytes, or quadrillion bytes), which is 2⁴⁸ sectors.

This has opened the door allowing ATA drives over 137GB to be released. Obviously any drives larger than 137GB would by nature conform to ATA-6. However, if you are installing a drive larger than that, you should also ensure that your motherboard BIOS has ATA-6 support.

BIOS Limitations

If your current hard drive is 8GB or smaller, your system might not be capable of handling a larger drive without a BIOS upgrade because many older (pre-1998) BIOSs can't handle drives above the 8.4GB limit, and others (pre-2002) have other limits such as 137GB.

Most ATA hard drives ship with a setup disk containing a software BIOS substitute such as Ontrack's Disk Manager or Phoenix Technologies' EZ-Drive (Phoenix purchased EZ-Drive creator StorageSoft in January 2002), but I don't recommend using a software BIOS replacement.

Ez-Drive, Disk Manager, and their OEM offshoots (Drive Guide, MAXBlast, Data Lifeguard, and others) can cause problems if you need to boot from floppy or CD media or if you need to repair the nonstandard master boot record these products use.

Operating System Limitations

More recent operating systems such as Windows Me, as well as Windows 2000 and XP, fortunately don't have any problems with larger drives. However, older operating systems might have limitations when it comes to using large drives.

DOS generally does not recognize drives larger than 8.4GB because those drives are accessed using logical block addressing (LBA) and DOS versions 6.x and lower use only CHS addressing.

Windows 95 has a 32GB hard disk capacity limit, and there is no way around it other than upgrading to Windows 98 or newer. Additionally, the retail or upgrade versions of Windows 95 (also called Windows 95 OSR 1 or Windows 95a) are further limited to using only the FAT16 (16-bit file allocation table) file system, which carries a maximum partition size limitation of 2GB.

Therefore, if you had a 30GB drive, you would be forced to divide it into 15 2GB partitions, with each appearing as a separate drive letter (drives C:–Q: in this example). Windows 95B and 95C can use the FAT32 file system, which allows partition sizes up to 2TB. Note that because of internal limitations, no version of FDISK can create partitions larger than 512MB.

Windows 98 supports large drives, but a bug in the FDISK program included with Windows 98 reduces the reported drive capacity by 64GB for drives over that capacity. The solution is an updated version of FDISK that can be downloaded from Microsoft.

Another bug appears in the FORMAT command with Windows 98: If you run FORMAT from a command prompt on a partition over 64GB, the size isn't reported correctly, although the entire partition is formatted.