Try putting the problem drive on the same IDE cable as the Benq. If the problem goes away, then perhaps the other IDE channel is in some way problematic at boot-up. That's assumign you hadn't used that channel until now.
If not, then it's something more complex to do with Windows or the BIOS struggling to recognise the two drives at once.
Does the Sony appear on the BIOS drives screen that shows at boot-up?