Recently my computer refused to recognize my external hard drive or secondary drive. I was about to have a real upset. Everything is on that drive. So while I was going over in my mind what was not on the back up drive I decided to try the main external hard drive on my laptop, worked fine so the problem was not the drive.
I shut off the laptop and went back to my desktop, still nothing. It would not even boot if the external was plugged in but I did get it to work in safe mode without the external, very curious. So I went back to a previous good date and rebooted. If I plugged in my external hard drive the computer was dead in the water. So I tried my laptop and the same thing was happening, two or three days later of fretting and after another update. Walla it worked so there must have been something wrong with the update.
Just in case you computer shuts down and acts weird.