I have a suspicion that something bad is going on with recent OSX Capitn updates. My back up drives have been flickering on and off both by direct connection and USB hub. They are 2 years old. Apple destroyed my WD and iOmega back up drives which meant that valuable and irreplaceable files containing my publications, writings, music and photographs were obliterated by whatever malware they included in the OS X system Snow Leopard or Lion. I willfully forgot the system. Purchased a PC laptop when I bought two new Toshiba, the Apple recommended back up drives, because of mistrust in a beloved Name and product. Link to my question in another thread about my particular issue so that if it is answered positive or negative you will have a good idea of what to do.
Linking saves a lot of time and effort when answers are pouring in from other Apple people. Apple always says it is you not me, so don't trust them, but do rely on the community who have solved through trial and error. Great community!