Some thoughts:
- Using a stripe of disks could be an option. Of course, loss of a single disk would be fatal for the backup set. Loss of a block would only affect a single file, (though not if in redundant metadata).
- You really don’t want DM-SMR drives.
- While another file system on the backup disk than ZFS might be desired, TrueNAS’ official support for such other file systems is non-existent.
- Using ZFS does allow scrubbing of the backup media to detect bit-rot and allows you to erase the failed file and write a new copy.
- Multiple USB drives, in different enclosures can be problematic. Both from a reliability side and potentially serial number side, (which can prevent them from working at the same time). See:
Why you should avoid USB attached drives for data pool disks
Some time ago, I thought about making a backup tool that sorted the files for backup based on size.
Thus, I could use multiple media, (back then, tapes & DVD-Rs), for my backups. It would backup the largest file first, on the first media and continue with the next largest files that fit until that media was full. Then start a new media with the largest file still needing to be backed up.
The benefit for that method was each media was independent of the others. Loss of one would not impact any file on another. (Unless a single file had to span multiple media… which I did not have thoughts on how to handle.)
This avoids the stripe of media problem.