This case highlights how a filesystem check can go wrong
This morning, I was copying files from one of the folders on the N4100pro to another folder on the same nas and hit an file system error.
Therefore, i did a file system check on NAS Admin page. The check took almost 12 hours to complete and when the nas restarted, my raid is gone.
The filesystem consistency check (and fix), does not normally makes things worse than they were. However, if the underlying RAID is somehow broken, or if the filesystem damage is real bad, the filesystem checker will trash the entire volume instead of fixing it.
These cases are usually not repairable. Filesystem check modifies the only existing copy of inodes (the key element of the filesystem) without leaving any copy of the original state. With the inodes broken, proper recovery is not really possible. You can get some bits here and there, but file names are gone.