Those symptoms usually point to a drive that is having problems either with its firmware and/or disk surface.
The best advice I can give is for you to:
Backup everything,
Download a copy of the WinDFT (assuming you use windows) and install it,
Switch off the NAS and pull the drive (not which slot it is in),
Fit the drive into a USB/SATA drive caddy,
Plug in the drive caddy (WinDFT should see it and allow it to be tested,
Run tests - if problems show up replace drive,
If no problems are reported reinstall drive into NAS,
Reformat drive in NAS and restore files and directory structure from backup.