Do other USB devices behave the same way or just this drive? Does it do the same thing on a different computer?
There are a few things you can try:
1) I would recommend creating a
Hirens Boot CD and utilize some of the options available to scan the drive for potential errors.
2) It could potentially be the computer turning off the ports to conserve power being used.
Not sure if you know how to check each of the USB Root Hubs to see if that is the case. Its real simple to do:
1. Go back to where you can see all the USB Root Hubs in Device Manager
2. Right click each one separately, choose Properties.
3. When window loads, click on Power Management tab.
4. Make sure that check box labeled "Allow the computer to turn off this device to save power" is unchecked.
See if that helps due to the computer turning the USB ports off after so long of not being used.