Partition D disapearing, and spontanously reapearing

Hello,

I run Windows 8.1 on an Alienware, and in the original settings I had 2 partitions, C and D. A month ago or so my partition D disapeared. And I do mean that it is nowhere to be found. I tried disc management, and even BIOS. It ISN'T there. It is not hidden, I looked it up, tried many things.

Then, after a while (a day, or so) it comes back under different names, E, F, etc, without any data damage. when I rename it back to D, all shortcuts work again.

WHAT is wrong and HOW do I fix it?