I am now checking if the problem is software and not hardware related. This post gave me the clue:
I was having the same error messages on my Moto E (stock Android 4.4.4). It will slow the phone down during wake-ups and also all of the app icons for apps located on SD card would disappear from the home screen. Very frustrating. In my case I traced it down to a single app that was causing it after being moved to SD card storage. It was the Amazon store app. While the phone is in the sleep mode it tries to perform some extensive file operations at the location it deployed to, which crashes the app itself and causes the OS/driver to lose the mount on the card. While it is obviously a defect in the OS, a workaround would be to move the offending app (in my case the Amazon store app) from SD card back into phone main storage. Hope this helps.While I don't have the Amazon store app, I can move apps back one at a time and see if that resolves the issue.
More later if I get any consistent results.