With you saying 'However, on doing a reboot, the system would not get beyond the initial stages of the POST - simply showing the header (BIOS revision details) and the processor, with the F5 and F8 options at the bottom. On unplugging the drive, and trying a reboot, it booted perfectly"
It sounds like your PC is trying to boot from the external hard drive. If you could, can you check to see which hard drive the pc is set to boot from. Could you also tell me how the PC's hard drive connects to the motherboard, i. e, SATA, IDE etc.
I own a WD My Book and the odd time if I have left it switched on an restarted the PC, my PC gets to the initial post stages and then will not continue. My PC's hard drive is connected to my motherboard by a SATA cable. I fixed the problem on mine by switching the PC off and then disconnecting the SATA cable off of the PC's motherboard and then reconnecting it. After I had done this everything was fine. Hope the info helps you Air_Man.