While trying to crop an image CS3 has started to tell me the scratch disk is full.

I currently have 2 120gb internal drives, 1 for programs and OS and the other purely for Photo storage.
I have bothed ticked as available scratch disks (OS drive has 23gb free and photo drive has 94gb free)
Ram usage set at 1201mb (max recommended by PS) with 1668mb available overall.

Any clues?

Terry