Why is 64GB SD Card "unreadable"?
Most Android phones format exFAT SDcards incorrectly, the BlackBerry Priv being the only one I know that does it correctly. All other Android devices create a partition of type 12 (sometimes referred as hexadecimal "c") instead of 7. The android device actually ignores the partition type, so if you set it to 7 then the sdcard will work fine everywhere.
There are many options:


You can format the sd card on your MAC, then use it on any other
device. Don't forget to copy your data somewhere else first, since
reformatting will erase the contents of the sd card.
Use the program fdisk on PC or Linux system. Use the command "mount" on the
linux system to see the name of the device corresponding to your sd
card (it looks like /dev/sdc1 ) then call fdisk on that device,
omitting the last digit (ex: fdisk /dev/sdc). Then use option "t" to
change the partition type to 7
I think there is a similar program on Windows.