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.
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.
If the cable isn't at fault, have a look at the energy preferences, specifically the setting for "allow disks to go to sleep whenever possible". I had this exact issue and had replaced the cable, internal drives, reformatted, repaired, re-done permissions and nothing worked.
Turned out that if the option to put drives to sleep when possible was unchecked and the drive had it's own firmware that put it to sleep, this would cause the drive to be disconnected without ejecting because the mac was expecting it to be kept constantly spinning.