You should have a tape drive for backup. Ideally, your tape backup should be able to image your entire disk. Choosing a tape drive used to be pretty complicated, with a plethora of different formats and media to chose from. It's much simpler now that the combination of cheap CD-ROM drives and huge hard disks has effectively killed off QIC and other sub-megabyte formats. These advances have also killed off a bunch of niche technologies for backup, including floptical disks, Bernoulli boxes, Iomega and SyQuest removable drives, and magneto-optical drives -- though the latter have survived as casual transfer media -- that is, as a sort of super-capable floppy replacement)
Some people like to burn CDs for backup. While this is a good way to archive precious data, the capacity is too low and the media cost too high for serious use. Rewritable CD-RWs do make a very nice transfer medium, however.
Digital Data Storage (DDS) capacities are a good match for today's multi-gigabyte drives (this is essentially the same technology as Digital Audio Tape or DAT). I'm told that Hewlett-Packard DDS devices are especially good, not surprising given HP's traditional obsession with reliability and overengineering stuff. All the DDSs I know about are SCSI devices.
At the high end, 8mm helical-scan tape (the stuff used in Sony camcorders) used to compete with DDS, but DDS drives have now outstripped it in capacity and transfer speed. A couple of other tape technologies (notably DLT) still do, but their media are extremely expensive.
Here's a quick summary of the major alternative DDS formats:
Table 2. DDS types
| Type | Gigabytes (uncompressed) | Gigabytes (compressed) | Speed (Kbytes/sec) |
|---|---|---|---|
| DDS-1 60-meter | 1.3 | 2-4 | 183-366 |
| DDS-1 90-meter | 2 | 4-8 | 183-366 |
| DDS-2 120-meter | 5 | 7-12 | 183-500 |
| DDS-3 120-meter | 12 | 24 | 700 |
| DDS-4 120-meter | 20 | 40 | 1100 |
DDS tape drives (and tapes) come actually in five variants: DDS, DDS-DC, DDS-2, DDS-3, and DDS-4. These are supposed to be downward compatible (e.g. DDS-2 reads/writes DDS-DC but not vice versa.) DDS and DDS-DC use 60m and 90m tapes; the -DC version adds hardware compression. DDS (non-DC) and DDS2 should be considered obsolete.
The DDS-3 and DDS-4 standards support higher density on the tape. DDS-4 is bleeding edge (high premium), but DDS-3 is coming down now, and can make the difference between single-tape and and multi-tape backups (which can often make the difference between daily backups and "why didn't I..." hand-wringing.)
Don't forget to put the cost of cleaning tapes into your budget. If a DDS drive gets dirty heads (determined by the read-after-write error rate), it will stop working until the heads are cleaned. DDS drives typically start complaining after about 50 full-tape operations.
SCSI tapes and DDS3 and below normally have the "narrow" 50-pin connector. If your controller is "wide" (68 pins) you'll need a cable adapter. Better yet is to put a cheap SCSI controller like an AHA 154x in the machine just for the tape -- otherwise you might end up plugging a tape drive with a single-ended electrical interface into a controller with a voltage-differential interface and dragging down the speed of every device on the bus.