My 250 GB I/O Magic USB external hard-drive randomly disconnects / ejects from the computer after between 2-30 minutes of use. When this happens, the blinking activity light on the front of the hard disk drive turns off; however, the disks can still be heard spinning. Unplugging and replugging in the USB does not reconnect the device and the activity light remains unlit. The only way to continue using it is to flip off and then on the power switch of the hard disk drive.
migrated from serverfault.com Dec 22 '09 at 5:31
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Sometimes Spotlight will cause this for some reason, but you said it also happens on Windows 7, so it leads me to believe it is a problem with the hardware.
Before considering to RMA the drive, try a different USB cable. It could be the External hard drive's USB port or some other internal component.