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Solving The Disk Drive Dilemma

Which is better: one big hard drive or two smaller drives with the same aggregate capacity?

It depends on what you're trying to do. If you're talking about IDE (ATA) drives of the same rotational speed in the currently popular capacities, you'll probably pay less for the single large drive. For example, two 120 Gb drives still cost less than a single 250 Gb drive, although the price difference isn't as significant as it used to be. The big advantage of having more than one drive is that it gives you redundancy. A secondary advantage is that you can significantly increase performance if you're willing to sacrifice some drive capacity.

With two drives you can protect yourself against a drive failure by using strategies such as a mirror disk, a cloned disk, or using the second disk purely for backup. A mirror, sometimes called RAID 1, simply duplicates data of one drive onto the other. It is not a secure backup (see Mirroring is not Backup) but it does provide a minimal level of backup, should your primary disk suffer a physical problem. By cloning your disk, creating an exact duplicate of your disk, you will have a backup should a problem occur, but again, this will be a point-in-time backup.

You can also use the second disk simply as a data disk where you can store a backup image of your primary system drive. Using an imaging program such as Acronis True Image, you can save a full image and incremental images of your primary drive to this backup drive. In this case, it is not necessary to have identical hard drives.

If you have two identical drives in your system, you can set up a RAID 0 array to increase the performance of the system by striping the data across both disks. Striping means writing part of a byte of data to one drive and part to another. When the system reads back the data, both halves are read at the same time, effectively cutting the read time in half. Although it increases performance, there is no fault-tolerance.

If you have four identical drives, you can employ both striping and mirroring to increase performance and provide fault-tolerance by having a duplicate of the drive; this configuration is called RAID 10.

Remember the price relationship only applies to drives with IDE interfaces in capacities that are in the middle of the current market. If you're looking for unusually small drives (say 40 Gb or less at the present time) or unusually large drives (more than 250 Gb), the cost advantage might not hold. Also if you're looking for smaller or larger drives with different interfaces, such as SATA or SCSI, the relationship could change. Since hard drive prices are in a state of constant flux, you need to check prices before you make a configuration decision.