Power went out during defrag, no space left

ahkjeldsen wrote:
I'll try run the defrag again. It was the first power outage in years, so I doubt it'll happen again.

Just to be clear in my post above I suggested expanding the volume then trying a defrag again.

ahkjeldsen wrote:
Like the md127 mount is placed at /data, but before the mount some data could potentially be stored directly at /data and thereby hidden from the 'du' command but not the 'df' command..

That's possible, but if that happened it would only explain less than 4GB. The OS partition is 4GB so with OS files already there....

ahkjeldsen wrote:
Do you know which drives they've used? Like I'm no hardware expert, so I just bought the drives I could find with the best reviews, the Red drives are optimized or something like that for NAS, so I thought that I would be safe - guess it's just a false sense of security then :/

The RED drives are good, but no drive is perfect. If you look at places like backblaze you will see that even the best drives can fail. Disks can and do fail at any time.

ahkjeldsen wrote:
Most of the files on the NAS is just media files that can be retrieved again from other sources if need be, therefore I didn't really see the need of a backup - though I'd still prefer not having to wipe the whole NAS as it would take plenty of time getting all the media files again.

Backing up and doing a factory reset (wipes all data, settings, everything) then restoring from backup is one option that I would suggest considering.