From my perspective it could be one of two issues.
1. The system is having problems with the motherboard. Which would explain why you are seeing both a HDD failure and a failure to test the memory.
2. While I have been told that Service Center 7.5 DOS testing should be able to address >4gb of RAM, there have been more than a few updates to our software since it's release.
If you would be willing, I would like to test if the new version of Service Center has the same problem for you. If it does then that means the issue is somewhere on the system (either RAM or Motherboard like I suggested), if not then likely the issue was Service Center 7.5.
Please PM me your email address and I will send you an evaluation copy of Service Center 8.