Windows 7. External SATAII to USB3 enclosure with external power.
Device mounts fine, and I can read/write files. After a few seconds of heavy read/write, the device will disconnect and I will hear the Windows "device unplugged" sound, followed the the plugged-in sound after a few seconds.
Edit: It turns out the enclosure, which is a Sabrent 3.5" SATA hard drive USB 3.0 Aluminum Enclosure, is the culprit. I bought it new in the box.