I have a couple of WD Green 2TB hard drives in my desktop.

Starting this week I have been prompted to run a disk scan on one every time I boot up my desktop, and have been getting both "cyclic redundancy" and "bad clusters" when trying to access the drive or move/copy files. I was able to repair the drive using CHKDSK and am in the process of recovering what I can, but is my hard drive on the way out? It's only 4 years old and I've had hard drives last much longer than this...

Am I able to do anything with these files with the cyclic redundancy error, or are they gone and lost forever?

If I need to replace it (and I suspect I do, as any warranty would be long gone by now) am I better to go with a WD Black? In fact what is the difference between WD black, green, red and blue drives?
Thanks in advance :)