I nearly say you have a problem with the main mother board. Possible controller chip but you would need an engineer to check. However Vista is a mine field and not as stable as XP. Vista can throw up a whole host of illogical problem. Try the link below and these people may be able to help you.
Whilst not promised to work this can force vista to have a rethink.
Firstly unplug both drives, then go to device manager which is accessed via control panel --> System and maintenance----> System----> device manager find your drives in this list and right click and uninstall.
Restart the system with the drives still unplugged and then plug them in again forcing vista to reinstall the drivers.
Failing that have a browse on the drive manufacturers website for updated drivers.