Tons of issues with SD cards and the S3
Are you running custom rom? stock rom? custom kernel? etc.
If you don't watch movies on the card - you can put it in your PC - backup all the data - and reformat it as Fat-32
Then you should have no issues
by default, the 64GB cards come formatted as exFat which has issues if using a custom rom and/or custom kernel combinations
LOTS of posts out there with this type of question
Simplest resolution is to reformat the card to Fat-32