Hi Stephen,
If you're receiving I/O errors just trying to map the virtual disk on the host, the cause is almost certainly a failing hard disk in your host. CHKDSK usually only does a superficial scan, and a problem that's isolated to the disk region containing your VM's data could easily trigger exactly the situation you're describing.
You may find additional clues in your Windows host's Event Viewer: Start> Run> eventvwr, then look in the System section for events related to I/O errors.
I suggest running an exhaustive disk test. Instructions are available on Microsoft's website. Be sure to select Scan for and attempt recovery of bad sectors... it will take much longer to run the check, but is much more likely to find (and maybe even fix) the problem.
Cheers,
--
Darius