For many years, digital tape drives were considered the only reasonable, cost-effective way to back up large quantities of data. They're still extremely popular in large businesses. Common digital tape formats include VXA and DDS (a data-optimized variant of DAT, digital audio tape). Although at one time tapes were notorious for losing data spontaneously, they have now achieved a comfortably high level of reliability and longevity. And in (extremely large) quantity, they can be quite economicalthough most of us will never get to the point where that economy of scale kicks in.
Tape drives have many virtues, but speed is not one of themat least, not for the lower-end tape drive most of us mere mortals can afford. It takes far longer to back up a given amount of data to a tape than to even a slow optical disk. Restoring files is even more time-consuming, because tapes must be rewound or fast-forwarded to the correct spot before the data can be transferred. And you will never be able to boot your Mac from a tape drive.
When truly phenomenal quantities of data must be backed up, when money is no object, and when time is plentiful, tape drives are perfect. High-capacity tape librariesautomated systems that can robotically swap tapes into and out of a bank of tape drivesare marvelous (and marvelously expensive) toys that form the backbone of many corporate backup systems. But for ordinary people with modest amounts of data, too little time, and even less money, they make little sense. Consider that you may spend about $1600 for a drive that supports 80 GB tapes, which in turn cost about $80 each. For that price, you could buy sixteen 80 GB hard drives or four 500 GB hard drives, which should be enough to provide speedy, redundant backups for all but the most extreme Mac setups.