Could be a lot of different things. How old is the drive?
Could be...
1) Hard drive failing
2) Something wrong with USB cord
3) Something wrong with the USB port, the cable connecting the USB ports to motherboard, etc
Quick troubleshooting options:
To troubleshoot 1) Try plugging it into another computer and see if it works fine. If it does, its probably option 3
To troubleshoot 2) Try a different usb cord...
To troubleshoot 3) Try a different usb port... if it works then it's probably something with that specific port. I had this just recently happen to me.
Also a less common problem is that the drive automatically powers down when idle and it could be doing this quickly for some reason. I also had this happen to me. You can...