How do i fix the Cyclic Redundancy Check error on my External Back Up Hard Drive

CA50 nailed it. CRC means the drive is physically going out. Back up what you can and format it before trying for a RMA (if formatting is possible. Use a low level format and zero out the drive. The bad sectors should still be visible to the manufacturer.) If it's more than a few years old, bite the bullet and get a new drive.
 
It is not necessarily the drive that is at fault, it may be the computer or the computers USB interface. If possible I would try it on another computer before trying a RMA. Also you could download the manufacturer of your hard drive diagnostic software to check the drive itself.