I don't know what caused it but lately when i try to do things with large files like compress them in a WinRAR file, it will say that the request could not be completed because of an I/O device error. What does this mean and how do i fix this?
This is not an external harddrive problem.
The drive hosting the files you are trying to compress has the problem.
If this is the external drive you mentioned then you may be mistaken.
Run a disk check on the drive to see if any errors are reported, then try again.
If you are not using a power cable on external drive you should do so.
Try using a different USB cable to connect drive.