I have this issue too... With my SanDisk card dismounting on my Samsung S3 (non-LTE) upon lock screen and mounting it again on wake.... It screws up the phone, the loading become slower, battery life becomes shorter, and funny error messages appear.
Tried to fix it twice at Samsung Service Centre (able to reproduce the error in front of the Samsung Technician, and he sorta replaced the card reader) but the same issue occurred.
In the end, I was fed up. I just went to purchase a Samsung SD Card at IT Fair (since even a reputable brand like SanDisk fails, I just go ahead and buy a brand not ususally associated with SD Cards, and a 32GB MicroSD card for $30 is kinda cheap, and promises high speed). Once done, the phone returns to normal. No issues, no funny messages.
Guess it's really a incompatibility issue between SanDisk and Samsung phones. The solution is not that expensive and solves the problem immediately.