This may explain..
This extract is from a review of a HDD enclosure......
"I ran into a major issue with this enclosure when I went to test it. It seems that the controller chip it is based upon (Genesys Logic GL811) conflicts with the USB 2.0 controller on my nForce2 motherboard. After some extensive research, much hair pulling, and a couple of late nights I was able to come to this conclusion. Even though this chip is supposed to be highly compatible it does not work with the onboard USB 2.0 controller on all nForce2 motherboards. What happens is the device recognizes fine, the drivers install properly and the drive shows up in my computer, but when you try and transfer data to it the device will disconnect. To be more specific, files under 20MB will transfer fine to the drive, but anything over 20MB will cause the device to say it cannot find the source file and disconnect. When this occurs though, strangely, the EHD-350 still shows activity occurring, even though nothing is being copied or written. In order to get the drive to show up again you have to disconnect it from the power source and the USB cable (which according to the manual is a big no-no when the activity light is on because your hard drive could be damaged) then wait a few moments and reconnect it. Let me tell you that it was a big and I mean big hassle. In order to get around this I had to use my USB 1.1 hub. Using the drive at USB 1.1 speed and transferring data is not very fun, especially large amounts of data. The problem continued on even after I purchased an add-on USB 2.0 card. With the add-on card the device would not recognize as a USB 2.0 device."