SanDisk Ultra 64GB SDcard

My 2GB sd card has no problem working in my iMac, in my PC laptop and in my Samsumg Android smart phone.

My 32GB sd card has no problem working in my iMac, in my PC laptop and in my Samsumg Android smart phone.

My SanDisk Ultra 64GB SDcard has been nothing but problems.
It won't work on multiple platforms.
Fine. In the end I decided to use it just as a bacup device on my iMac alone.
Problems: It keeps unmounting itself and then there a faff to remount it.
What a palavar!