I would be very concerned that the drive may be failing, and make copies of anything important.
Is the external drive full? Go to My Computer and right click on your external drive and click on properties and it will tell how much is free and used. 90% is very full. Check all your hard drives while you are at it.

You should go to the website of your hard drive maker and download a diagnostic utility to check the health of your hard drive. It will be on the website under support. This must be run from bootable floppy, bootable usb, or bootable cd.