Help ASAP!!!

* I have a MacBook 2011 model but just upgraded the operating system

* I also have SanDisk Ultra 64 Gig Micro SD XC

* MacBook is reading the memory card but I’m not allowed to save anything on it

* I’ve tried reformatting my memory card but it won’t let me

* I have tried it on a Windows laptop and it let’s me save my work

Any ideas on the problem?

Kind regards, Mark