I have just lost half my data. TLDR; The power went off, I have a UPS and I had it set to turn off as soon as the power is lost, but as it turns out the setting had reverted to the shutdown with 10 min remaining and it must have cut. When it started up some disks didn't mount properly?? I have 7 with 5 and 2 parity. 3 drives were not mounting, i managed to get 5 mounted 3 disks and the 2 parity and did a parity check and it passed. I thought cool my data is good, however I couldn't access 2 of the emulated drives which was a bit odd but i thought that must be how it is as it was a while ago that i tested a failure out. I then re added the 2 disks in expecting it to come up as rebuild, it came up with check parity but it had write to parity checked. In my memory I thought it was supposed to say rebuild disks or something to that extent but I assumed i was mis-remembering it and it would just rebuild the disks from parity. anyway after i did it all the disks were active so i thought it was working but after it finished there was still no file system and I couldn't access the data on the drives.

So I'm not sure what happened but 2 of my disks that were full are gone and i'm not sure what unraid was doing. I think it could have not started the array if disks were missing initially. It could have been more clear about exactly what it was doing when it was resetting my parity, i didnt want that i didnt want it to touch parity. maybe even have options to write lock drives in fail modes or the user can lock them. that way i know if i try something my data is safe. All my super blocks were gone. Was that the power failure or was that unraid when it started up? Seems like I would have been better off with ZFS because this is the exact scenario that I picked unraid for and it didn't work.

Any advice as to what happened? So I can avoid it in the future.

Edited March 4Mar 4 by Bladefire999

First it is possible the disks in question just have a damaged file system which is stopping them mounting, so you could try to see if Filesystem Check helps. Ideally when one has an unmountable disk it is better to do this before attempting any rebuilds as sometimes the original drive is in a better state than the emulated one.

It is worth remembering for future reference that whenever Unraid says it is emulating a disk, then whatever shows on the emulated disk (including an unmountable file system) will be what you end up with if you rebuild the disk. The rebuild process has no concept of data - just of the bit pattern it thinks should be in any particular sector.

Failing being able to repair the file system it could be worth seeing if a file recovery utility such as UFS Explorer on Windows can recover the data. That software is not free when trying to recover data but it does have a free option that shows what would be recovered before you have to commit to paying anything.

If it happens again, recommend asking for help and posting the diagnostics; most likely that was a recoverable scenario.