I had a similar problem in that I kept losing all the drives for the multi card reader on my pc.
After a lot of hunting about the excuse turned out to be a build up of static in the machine?? Anyway the cure was to shut the pc down, turn off the power switch on the back then hold the start/power on button on the front for 30-45 seconds. It started up just fine after that and I've never had a problem since.
Might be worth a try?