I had this same issue with a Class 10 SanDisk Ultra 32GB microSD card on my Galaxy S3.
my card seem no error after check by windows 7 utility but offen unexpectedly removed in Android.
I guess the problem issue because android has updated the os with patch, that why people (myself included) get to use the card happily for 3-4 months (approx) until it starts having problems.
i need the action from sandisk to fix this, it drain my battery to emty so fast.
