If I recall -- I'm rusty as hell on IPX -- The system syncs the HDDs as RAID 0 (identical copies). So if one HDD is failing, it might do what you're seeing. You should also see errors that the system can't sync the HDDs.

Like I said, rusty as hell on IPX... Formatting the HDD, or replacing the HDD would probably be my best guess for a solution. It does need to be an NEC HDD, according to old Georg the trainer out of Santa Clara, back in the day.