Short answer - we don't know. Not enough info. What can you validate?
Going on the limited information provided, it may be a chipset error, a disk error, cable. I had a system once which couldn't drive the ide bus. When warm I would occasionally get on stdout
hda: not ready
hdb: not ready
and it would sit there, unable to shutdown or anything. Shutdown was, after all, on the disk. To judge by the times
Dec 9 16:25:58 Myserver kernel: ide: failed opcode was: unknown
Dec 9 17:01:15 Myserver syslogd 1.4.1: restart.
something similar may be happening.