Are you running any add-ons like fun_plug or packages? How many drives do you have and which mode are you running them i.e single volumes, RAID etc? It might be bad sectors on a disk causing the drive to be unresponsive during attempted recovery. But this all depends on your drives, their configuration and how your NAS is configured to use them.
I would suggest shutting down the NAS and testing your drive(s) on a PC using the manufacturers diagnostics if you can.