what operating system is this on, and what do the bios say it is, you may need a bios update, if you have a olderboard and a older bios version, it may not be able to detect the device for the simple reason, it does not know what it is.
I'm on XP and the BIOS definitely can identify the drive, I've had it plugged in on the direct IDE before and it worked fine. The problem is now it's in the firewire case so it's swappable between my mac and PC, that the PC's not recognising it.