It's worth mentioning that disks that have had a long and active life (e.g. maybe the 4/10GB disks Ivanerrol's referring to) are probably more likely to fail during storage than ones that have had much less use. For a start the lubricants will be in different condition.
I back up my files to external disks which are stored off-line, but those aren't my only copy. I have two (I should update it to three
) sets of disks. One stays online all the time connected by Firewire, and I regularly plug the backups in (typically via USB2) and resynchronise.
So far I've got a small enough set of disks (and each one is big enough that it's likely that SOMETHING on it has changed) that I'm not tempted to leave the backups on the shelf for long periods. I haven't noticed failing offline disks yet, although I'm sure it'll happen at some time.
I have several exposures to loss at the moment:
- Failure of a primary disk (or accidental deletion/etc) before the files have been copied to the secondary.
- Failure of a secondary that's only discovered when trying to recover files from it.
- Catastrophic failure while the secondaries and primaries are being resynchronised. This is mitigated slightly by only connecting and synchronising one secondary disk at a time).
Having a 3rd copy of the files will reduce these risks significantly, but even as it is I have a reasonable level of protection. Having hundreds of DVD-Rs is not an attractive proposition for me (especially as these will require regeneration over time). CD-Rs are generally better for longevity, but still require regeneration and will number in the thousands. I retired my CD archives a few years ago when it got unwieldly. A 2nd set of secondary disks is the path I will probably go down, although right now I've just had to add another primary+secondary so need to build up the funds again and weigh up the risks.
Having only one copy (on a hard disk) of my files would scare me, no matter where the disk is stored.