The CMOS back-up battery would be the obvious candidate. Especially iof the machine is a few years old and spends a lot of its time switched off. What about the system clock - has that played up at all?
I had a similar problem on my old P-100 at home (on mine it was the HDDs that would not read!) but re-detecting the HDDs seems to have fixed the problem (although we were warned of a power outage this morning and I switched off before leaving for work - hope it boots when I get back home!)