First double-check that it isn't already resyncing.
If it's not, you can pull the disk, and connect it to a PC (either USB or SATA).
With Windows you can either unformat the disk (use Windows Disk Manager, and delete every "volume"), or zero it with WDC Lifeguard or Seatools (the "quick zeros" test in Lifeguard is enough). Then hot-insert it into the NAS. It should automatically resync if you are using XRAID.
I don't know how to unformat with a Mac, but you can alternatively just format the disk and then hot-insert it back into the NAS. From there you'd select the disk on the volume page, and reformat it. After that it should automatically resync.
