How long have you had it exactly?
This is either one of three things:
1) Failing drive
2) Bad USB cable
3) Corrupt mass storage driver for this device
Try this in order:
1) Try using a different USB port
2) Try another cable
3) With the drive plugged in, assuming you can move the mouse, double click on the little green arrow icon on the bottom right of the screen near the clock and "safely remove" the drive from Windows.
4) Get a hold of a drive diagnostic or drop by a trusted local repair shop for analysis.
The good news is that it is likely still under warranty if less than a year old. The bad news is that if the drive is too far gone, recovering data may be impossible or prohibitively expensive. Also, sending the drive in for warranty and dealing with the hassles may not be worth it vs. buying a new external at today's bargain prices.
Good luck!
