Only way to verify what is going on is to remove it from the 500X and connect it directly on a PC Sata cable.
Could be all kind of things:
- defect disk
- issue with the filesystem on the disk
- issue with 500X mainboard
You could also try to connect over the netwerk by telnet as described here to verify if you see the disk:http://www.mede8erforum.com/index.php/topic,701.0.html