Disk Utility does not recognize internal HD

Hello community!

I am a computer lab assistant (all iMacs) for my work and am troubleshooting a problem as follows:

Specs:

Mac A:

OS X 10.8

2.7 GHz Intel Core i5 processor

4 GB 1333 MHz DDR3 memory (2 slots x 2 GB in use, 2 slots empty)

27-inch Mid-2011 iMac

Graphics: AMD Radeon HD 6770M 512MB

The problem is that Disk Utility does not always recognize the internal HD on mac A. In fact, only once it showed up recently and that was after a couple of restarts. Programs run very slowly and are not very responsive.

Troubleshooting steps taken thus far:

1. restarted mac A in internet recovery mode and ran Apple Hardware Test about 8 times (quick test 4 times and extended test 4 times). Every time it hangs indefinitely on the memory check.

2. Installed and ran memtest in single user mode (mac A) with arguments "all 2" to run the test twice. All tests passed.

3. Started up mac A in Target Disk Mode and booted mac B via firewire. Everything runs fine (fast), but disk utility installed on mac A's HD does not recognize any HD (neither firewire nor internal).

4. Started up mac B in target disk mode and booted mac A via firewire. Everything runs fine (fast). Disk Utility on mac B's HD recognizes both firewire HD (of mac B) and internal HD (of mac A). Disk Utility verify and repair completed on mac A's internal HD. Everything OK.

I'm unsure how to proceed. I assume the RAM on mac A is fine because it runs mac B's HD via firewire very fast (it handled opening all the apps on the doc simultaneously relatively well). I assume mac A's HD is fine because via firewire it shows up in disk utility and runs very fast on mac B. I just don't understand why Mac A's HD won't work well on mac A. Could it be a loose SATA connection between the HD and the logic board? Would that cause these symptoms?

Thanks for any input.