Interestingly enough there's a comment here:

http://www.toad.com/gnu/sysadmin/#usbpower

that links to this thread, and observes that enclosure may not provide adequate power for some drives. It didn't produce the symptoms mentioned here, but could be worth looking into.

To further NooNoo's reference to power management, it may help us to know if "Disable power status polling" is checked on the Settings tab in Properties for Advanced Power Management support in System Devices. (Takes deep breath...)