Windows 7: Hard drive not recognized

hey, i am trying to backup files and programs to an external hard drive. and a messages comes up saying "one of the USB devices attached to this pc has malfunctioned and windows does not recognize it. can anyone help? thanks

Usually this means there is a hardware issue.
Can you try the external HDD in another PC and see if the issue occurs there?

If not, then you may want to reformat it and then move it back to your PC and see if the problem happens again... if it does, then you may have a USB issue on your machine.
Try another USB port if so, and if that doesn't do it, try reinstalling your USB device drivers for your PC.

Though 9 out of 10 times this is an issue with the device which may be remedied by moving to another computer, or the device itself is actually damaged/defective.

This problem can occur for a number of reasons and you may have work out a number of fixes as may be applicable in your case.

This Microsoft Support article details the various causes and fixes that may help.

Before trying these, do power cycle your PC/laptop ( Remove mains power and battery pack. Reconnect after a few minutes and reboot your machine.) In many cases this simple process has helped many in resolving the problem.