I just had exactly the same problem ("An unknown error occurred while accessing H:\CurrentSetup.xml") again four times in a row, even though everything worked fine (following the steps from my other post on 250GB drives) a week ago.
Here are the two things I noticed and fixed, one or both of which seem to have fixed the problem:
1. Windows did not mount the DMS partitions in sequence. I had p:\phtsys and s:\phtdta. That's definitely not right, so I used Windows Disk Management to change the phtdta drive letter to q:\phtdta. (I don't know why this happened but could be because I occasionally use virtual drives.)
2. Autoplay was disabled on the hard drives. I've seen various opinions about this, but from my experience when the autoplay dialog pops up after I attach my DMS, I know it's going to work. So, I re-enabled autoplay.
After doing both, I rebooted, and following the usual procedure (start PMM first, then insert DMS) it works fine again.
So, InterMurph, you might want to check to see if you have either one of these issues and correct as needed. This might get you going again.