When all is said and done, your backup is useless if you’re unable to restore files or even entire systems when the need arises. But beyond that, you also need to have the confidence that should you ever need to turn to a backup, it will be there and ready to go.  

Therefore, it’s important to consider the following factors:

Regular Checks
With any archived data frequent testing of restore capability is recommended, if not essential.

Shelf Life
Compliance and other regulatory retention requirements can call for data to be kept for years, so you need to ensure that the storage medium selected has sufficient expected shelf-life. In general, tape life expectancy is between 4 to 6 times that of disk (in fact, media manufacturers for LTO tape specify expectancy of up to 30 years!)

Efficient Restores
While the random access of disk supports fast file access for restore, for batch streaming restores of large amounts of data, tape may prove the most efficient. Powerful software indexing can also facilitate individual file restore from tape.

Disk May Be Necessary, But is Not Sufficient
Disk usage has been expanded via new technologies such as mirroring and replication, however if something were to affect the original data (error, system failure, virus, hacker), the problem could be propagated to your backup. An off-line copy of data is needed that is not susceptible to on-line corruption. Tape provides a secure, offline, system isolated copy of the data – and the ultimate recovery mechanism.