USB Ejecting Itself

i have been having a problem since about 10.6.4 (maybe earlier) and i am on 10.6.7 now with no change. It seems that no matter what i try to do i can not get a external usb drive (hard drive, flash drive, android phone, ipod, logitech harmony 1100 remote, etc.) to stay on any of my macs. I have a Mac Pro 1,1 and a Macbook Pro 4,1. The drive will stay mounted forever, but as soon as i try to move files onto it it will run for about 1 minute then eject itself. i have reinstalled all the software on my Mac Pro, tried different usb ports, different drives, etc. I know it is not the drives because i can use them in both Parallels on my Mac Pro or Bootcamp on my Macbook Pro.

Any ideas?

Kingpin