It seems from what I have read in different forums that you could have one of three problems.
The first being that you have very little free space left on the drive. This seems to be the most common cause of the IO error message according to those who have posted.
Second it could be that the hard drive is failing. Download and run this smart monitor to check the health of your HDD:
Third could be a problem with your port on your computer. I wouldn't think this was the cause. But, if you started getting error messages as soon as you moved it to the second port then it's worth checking into.
Of course, there are other things that it could be, but from others' problems these look like the things to check.