Besides optical discs and magneto-optical disks, you can find many other removable storage devices, from a wide range of manufacturers. The most popular onesand, for our purposes, the only ones potentially worth consideringare made by Iomega.
Iomega Zip and Jaz
Iomega Zip drives store 100 MB to 750 MB on removable magnetic disk cartridges that are slower than hard drives, but usually faster than optical discs and much faster than tape drives. Although the cost of media per gigabyte is comparatively high, Zip disks can be reused indefinitely. The same is true of the now-discontinued Jaz drives, which support 1 GB and 2 GB Jaz disks. Unfortunately, Zip and Jaz disks have a reputation for being unreliable, so I recommend against using them for backups.
Iomega's latest removable-storage device, REV, uses rugged, hard disk-based cartridges that hold 35 GB each (www.iomega.com). Designed as a faster and more robust backup platform than tapes or DVDs, the FireWire version of this drive even includes a free copy of Retrospect Express. Although REV is significantly slower than ordinary hard drives, the cost of a single drive (about $300) plus several disks (about $50 each) could be less than the cost of two or more stand-alone hard drives.
The only significant downside is that 35 GB is not large enough to hold the contents of some users' startup volumes; although you can certainly split a backup onto multiple disks, this would prevent you from making a bootable duplicate. However, if you have well under 35 GB of data on your startup volume, REV may be an excellent choice. A word of caution: REV is a recent and relatively untested technology. Given the poor reputation of Zip and Jaz drives, it might be wise to wait for evidence of reliability before trusting your data to REV.