It is likely that your Data disk may be dying or at least have it's filesystem corrupted (which, again, is often a symptom of the disk dying).
Firstly make sure you have backups of all important data on that disk.
Then you can check S.M.A.R.T. status of the drive and run self-checks. If the checks say the drive is OK then run a filesystem check utility, depending on the type of filesystem you have on that drive.
Read the wikipedia article
Read Ubuntu wiki
Run "long" or "extended" test on the drive with
sudo smartctl -t long /dev/sdX, wait for it to finish, see if the test completes successfully, possibly paste the output of
sudo smartctl -a /dev/sdX here