Recently I ran into a very strange phenomenon with one of the servers I manage used to serve samba shares over the local network. It was running Ubuntu Server 8.04 and I decided to upgrade the distribution. After painstakingly pushing one release at a time I ended up with version 10.04. The server threw an error with mounting one of the drives (sdb1) so I edited the fstab so it used the drive's UUID instead of an absolute path. After remounting everything the original contents of /dev/sdd1 were missing and were replaced by files that we thought we lost 2 years ago.
Here is the list of drives on the system (sda2 is logical containing sda5 which is swap)
ls /dev/sd* /dev/sda /dev/sda2 /dev/sdb /dev/sdc /dev/sdd /dev/sda1 /dev/sda5 /dev/sdb1 /dev/sdc1 /dev/sdd1 df /dev/sd*1 -H Filesystem Size Used Avail Use% Mounted on /dev/sda1 487G 459G 2.9G 100% / /dev/sdb1 493G 433G 35G 93% /srvnew /dev/sdc1 501G 28G 448G 6% /srvb /dev/sdd1 1.5T 112G 1.3T 8% /srv15
As you can see there are four hard drives in the system and are all mounted. There is no logical reason why the data of sdd1 should be swapped. Because I am working remotely over SSH, I made the guys that own the server to physically open its case and confirm there are four hard drives installed :)
Another strange thing is that they reported the fourth hard drive was also 500GB and not 1.5TB and nobody swapped drives.
Tried running data recovery on each drive which returned no files (expected as no files are ever deleted from the server). I am really confused and have no idea where to start from.