Can't mount DMG files or recognize external drives

I have spent much of the last 2 weeks searching for a solution to this (these?) problems. I am running 10.6.8 in a mid 2010 13" MB pro. One day I noticed that it wouldn't recognize disks in the optical drive, then I checked to see if external drives would work, which they would not. I tried a number of things, zapped the PRAM, reset power manager, repaired permissions, and eliminating some files (as several posts suggested), and nothing worked. However, it would boot into target disk mode and the hard drive was recognized. I backed up everything, ran disk utility and diskwarrior (both of which repaired a host of not significant issues). As I could not get it to mount the install disk, I could neither run a hardware check nor archive and install. It seemed as though it must be a motherboard issue, so I picked up a used 1st generation Macbook and put the drive in there in preparation for sending the Pro (motherboard) off to be checked by a tech guy I know. The new machine then exibited all of the same problems, so now it seems like it is a software issue. I have repaired permissions multiple times and tried to use several different programs to open DMG files, but nothing worked. I used target disk mode to replace the diskmount.app on the drive in question with a fresh version from the original install disk, and again, no love was shown. Now I need help. The error message I get when trying to expand the DMG files is "device not mounted." Duh! I searched these and other forums thoroughly, and the only similar issues I found were for older OSs or were never resolved.

HELP!

Thanks in advance!