SATA HD stopped being detected, with a one time exception

I'm relatively new to anything DIY with computer hardware, and I'm having an issue with my first custom built desktop. I had an old desktop with a usable HD, so I've used that in this build. It's a 320 GB Western Digital SATA HD, and it's been working since I put the computer together at the end of July (2012). Recently, upon booting, the BIOS doesn't recognize the HD. It's about 50/50 on whether the BIOS recognizes that something is connected to SATA port P0 or not. Sometimes it pops up with "P0: " as a choice to try to boot from, and other times, the only choice is the DVD drive.

I've tried switching out the power cords and SATA cables with the DVD drive. The results of swapping cables and ports on the mobo were that the DVD drive always worked, and the HD never did. This leads me to believe that it is an issue with the HD.

I've tried removing the mobo battery to reset the BIOS (I have no custom settings, so this would reset anything that was accidentally changed, possibly by another user in the house). Still no change.

I've tried booting from the Windows 7 disc in the DVD drive, but as expected, the disc cannot repair or re-install the OS without a recognizable drive to do so.

Now is where things get a little weird. After maybe 3 tries at using the Windows 7 disc, I removed the disc between boots, and the computer booted right up from the HD. Having no clue what I did to fix the issue (the only change I made between boots was remove the disc from the DVD drive), I just was thrilled that it was working. I checked a few files and opened a browser to make sure things were working, and then shut down the computer and removed the power supply to put the case back on the computer. Afterwards, the HD has been unrecognized since.

Here are my system specs:
Mobo -- ASUS M5A97 AM3+ AMD Motherboard with UEFI BIOS
CPU -- AMD FX-8120 Zambezi 3.1GHz Socket AM3+ 125W Eight-Core Desktop Processor
HD -- Western Digital 320 GB SATA (if you need something more specific, I might be able to decipher more from a serial number)

If there are any other specs that would be relevant, let me know and I'll provide them.

Thanks for any helpful ideas on what to do next.