When I upgraded the internal drive in my MacBook Pro, I put the old drive in a USB to SATA enclosure, based on the JMicron 2338 chipset. I find that in heavy usage, the drive will lock up with the access light on steady and my computer will hang. In fact I had to force reboot twice tonight just trying to copy a large Parallels VM to that drive. I finally gave up and used a Firewire drive instead.
A google search turned up several reports of hangs and other problems with that chipset. Unfortunately all SATA to USB adapters I’ve seen use the same chipset.