One of my hard drives is dying, I need to replace it but I don't know how to physically tell which is which. I know the dying one is the one that's nearly full but I don't know how I'm going to tell which one that is once I open up the case.
Download HD Tune and check the Info tab. It shows you the partition size(s) and the serial number. Write down the serial number and check it on the label of the hard drive.
You can also check the SMART parameters or perform a surface scan with HD Tune to determine if this hard drive is dying.