After I left the BIOS, it showed up in the operating system. I didn't have to make any modification to the BIOS settings. I guess going into the BIOS "reminded" Windows there was another drive there? All is well now, but I'm left wondering what the dynamic was.
maybe just loose connections. It happened to my core 2 quad pc alot. Check to see if your cabinet's side panel or any other component is touching the sata cable or the sata power cable just to make sure to ensure it might not happen in the future.etc.
Or, maybe your HDD is failing?(maybe not, but just a guess) you can install a program called HDSentinel on windows and monitor your HDD's performance % and health % over time. Just install the program and use the PC normally and see the HDD's health and performance status after few days of normal usage of your PC. If it's decreasing, it maybe loose connection or your HDD is failing over time at the moment.
(Note:-You can just install the HDSentinel software and use ur PC like normal, you dont have to click and start any tests, it will automatically monitor your hard drives while your using it overtime, as long as you keep the HDSentinel background service running).
Good Luck~!