External drive always flashing

I have 3 external drives attached to my iMac. A thunderbolt, a USB mini and a G-Tech FW800. The light on the G-Tech blinks all the time, showing the drive is being accessed. There are no programs running and no auto back-up or TimeMachine set. But every few seconds the light on the drive flashes. I can walk by the office at night and see the light flashing away in the dark. I can't figure out what's doing it. I checked Spotlight and there is no inventory going on, or whatever you call it. Nothing seems to be running at all, yet the drive keeps flashing and I can hear the clicking sound. Any ideas?