October 24, 201510 yr Right now I copy data to my backup UnRaid server using a program called Free File Sync. If I lose one drive, I understand that when I replace it, it will be rebuilt with parity data from the parity drive. If I lose two drives, say at the same time, will the first drive be rebuilt with parity data? What happens to the second drive, obviously I have lost its data, but when I replace the drive, I assume UnRaid treats it like new drive? I would just use my Free File Sync program to compare data and copy over the lost data from drive two, is that correct? If I have a 5TB parity drive and lose two 2TB drives at the same time, can the parity drive not rebuild the second drives data too?
October 24, 201510 yr If you happen to lose 2 drives at the same time, then the files stored on those drives will be lost, with unRaid unable to rebuild either of them. However, all of your other data drives will still be intact. (unlike say RAID-5 where ALL of your data would be lost in that situation). But, if one of the drives lost happens to be the parity drive, then only the files on the data drive would be lost.
October 24, 201510 yr If I have a 5TB parity drive and lose two 2TB drives at the same time, can the parity drive not rebuild the second drives data too? No. The parity drive doesn't store any data unique to any single disk. It's only usable if all drives but 1 are readable. The instant the second drive goes offline, both failed drives are non-recoverable. See here and here for more information.
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