Class 10 64gb MicroSD cards come formatted as exfat and as it turns out, a hidden partition. This leads to easy corruption and difficult conversion. I recommend class 6 64gb MicroSD cards. They're getting hard to find but I've purchased several for my Android tablets from this ebay vendor:
I ended up with one class 10 64gb Microsd card through a dispute. I found the solution in the XDA Samsung Gaaxy S3 forum in the following thread:
Samsung Galaxy S III I9300, I9305 > Galaxy S III Accessories > 32GB or 64GB MicroSD cards for Galaxy S3.
32GB or 64GB MicroSD cards for Galaxy S3 - Page 6 - xda-developers
The first post outlines the problem and continues with a complex solution using windows 7:
"IF OWN THIS SANDISK 32GB OR 64GB C10 UHS-I AND YOU ARE HAVING ISSUES WITH THIS CARD OR IF YOU WANT TO RECLAIM YOUR 16MB BACK SEE THIS POST!
This applies to the SanDisk Mobile Ultra 30MB/s 32GB or 64GB Class 10 UHS-I cards (the Class 6 cards did not have this issue (I believe)
To everyone - you may have a problem with this SanDisk C10 card in your Android device (especially the Galaxy 7.7 and S2).
Not because its C10, not because its UHS-I, but because of SanDisk's making 2 partitions.
edit: it may be that both the 32GB and 64GB C10 cards both have 2 partitions. you may have to do this for the 32GB C10 card also.
The old 64GB C6 card just had 1 partition, one large 59.48GB (after format) drive" You'll find the more complex method in this thread.
In the same thread another user suggested the free windows program: MiniTool Partition Wizard Home Edition.
MiniTool Partition Wizard Home Edition - CNET Download.com
It allows you to delete the sd partition, zero it out, re-create the partition in either Fat32 or NTFS, format and you're good to go. Make sure you have the sd card partition selected or you could nuke your computer.
I was able to convert the card to NTFS and see it recognized in my Asus TF201 tablet. My Galaxy s3 couldn't see the card as NTFS so I then deleted the partition and re-created it as Fat32. For those who need to use Fat32 for rooting or prefer NTFS, like me, MiniTool Partition Wizard Home Edition works.
I'm not a fan of exfat and didn't test putting Humpty Dumpty back together again but I assume it could be reformatted to exfat if one wished. I'm still sticking to class 6 cards.