There are two ways this could be done, that come to mind, with all drives in one box.
For good fault protection and ease of restoration:
- UnionFs one 10TB drive together with one 6TB drive, for a total of 16TB.
- Do the same with the second drive pair.
- Rsync UnionFs 1 to UnionFs 2
(Other than the extra piece of combining 2 drives into a UnionFS drive, the Rsync command line is outlined in the guide. It would be a matter of setting up the command line with UnionFs1 as the source and UnionFS2 as the destination.)
This give you 100% data backup and restorations are easy. If something in the primary pair fails, simply re-point your shared folders to UnionFs 2, with identical content, and you're back on line. (This is in the guide.) However, there's no bitrot protection.
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For the maximum disk real-estate, with protection and recovery options:
- UnionFS all drives together 10+6+6, saving one 10TB drive for SNAPRAID. (For SNAPRAID to work, the parity drive must be equal to, or larger, than the largest drive in the array.)
- This would give you a UnionFS drive of 22Tb
- SNAPRAID provides decent options for the recovery of the loss of a drive, scrubbing for bitrot and more. Take a look-> here.
While getting started is easy, recovery is more involved than an Rsync backup. ((In any case, when recovering, it's time to slow down, take a breath and do some reading before doing anything, in any case.))
The trade off is more disk space and bitrot protection, versus ease of restoration.
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I'm not telling you what to do, but it makes sense to investigate the options before committing. And remember, as you consider the your options; it's not a matter of "if" a drive will fail, it's a matter of "when".