If a fix operation started and a disk failed, it will fail out and you will have to start over.
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r/techsupport
If a second data drive dies during the "snapraid fix" operation for the first failed drive, will the fix for the first failed drive continue without issue?
If a fix operation started and a disk failed, it will fail out and you will have to start over and fix both disks.
Similarly, if one of the two parity drives fails during a sync operation, will the second parity drive allow the operation to complete successfully without any interaction from the user?
No. It will fail out and you will have to rebuild the new parity.