I hope you mean 'one master drive + two or more cloned backups'.
I would like one large drive to store all my photos long-term. Note, not a backup of your archive (which might only replicate errors).
If you would find that onerous and want to reduce the risk, you might consider create two identical archives on two separate disks, updating both at the same time. If you can recreate your archive from the (backed up) sources with a reasonable amount of work, then one disk is probably an acceptable risk. Think about what you would do when (rather than if) your archive disk fails. Your only defences are regular integrity checking, backups and multiple copies. Disks of any make/type can fail at any time, sometimes it's not immediately obvious, some errors may be automatically recovered by the disk itself (bad block remapping), silently, with or without file corruption. Just make sure you always dismount the drive before disconnecting it and you should be fine. More to the point, how reliable is it for long-term storage? Or is longevity based more on the physical qualities of the drive?
