Hard drive dying?

Hey everyone—I’ve run into a bit of a headache. I have two separate drives set up here; one runs my Windows OS and the other is dedicated strictly to my games, movies, and various media files. Everything was functioning perfectly fine until today when—and this is almost comical in its timing—I tried to launch a game, and the client just started glitching out on me. So, naturally, I rebooted the machine, but now the system isn't even acknowledging the existence of that second drive. I’ve even tried the whole "unplug it and plug it back in" routine, but no luck whatsoever.

On a side note—and I can't quite put my finger on why—the boot sequence has become incredibly sluggish lately. It hangs for an eternity at the BIOS screen before finally moving on to the "loading operating system" phase. This definitely wasn't happening a few weeks ago.

To be honest, I suspect there might be some hardware fatigue involved; the casing felt noticeably warm to the touch when I went to check on it...
The drive is completely invisible to Hard Disk Sentinel now...

If you can't spot it in the BIOS, then the hard drive has likely kicked the bucket—or, if we're feeling optimistic, maybe just a faulty SATA cable.
Since you have another working drive on hand, why not try swapping out the SATA cables? I wouldn't bet the house on it being the cable, but hey, it doesn't cost you anything to double-check, right?