I have two usb external hard drives connected to my XP desktop. Neither external has an on/off power switch, so they're automatically started every time the computer is booted. Is there any way to be able to turn them on/off manually?
Are the disks powered from ac power via dc adapters, or are they powered from the USB ports? In either case, you can connect/disconnect them from the power supply according to your needs.
I agree with Robin. Unplug them when not in use. That will save in electricity costs, and wear and tear on the drive motors. If you need access to these drives daily, you might consider installing them internally to the PC.