After you had finally gotten your very first 64GB SD card, you will eventually hit this problem sooner or later. Yes, everyone, unless you are the type who is not concern that your tablet just doesn't work with your card or simple could stand how slow the card is somehow.
The major problem with exFat is that its a more Windows format and it's teh default format for formatting SDXC card anyway, even from the official SDCard.org:
But most tablets are Android based and as such more towards *nix and somehow exFat just doesn't play well there.The problem will start anytime from not detecting the SDCard at all to very slow performance especially when many files are involved. In some extreme case, its even because there are 2 partition created and it start to seriously confuse the OS. Just search "SDXC exFat Problem" and you will know what I mean.
I know its probably not right, but I recommend to keep your 64GB SDXC on Fat32. And Microsoft definitely does not agree with me because they had disable such a feature on their default format utility. You can only format it to exFat or NTFS, which neither works well on Android. But of course there is a easier way here:
Make sure you read through and decide on whether you want to use the DOS utility or the GUI one. Just format it to Fat32 and suddenly many of your problem is gone.