I have a class 10 64gb San Disk ultra and it has worked just fine since the day I put it in my pro. No disconnect issues or anything. I leave it in there. No need to take it out.
I agree with oin. If you were determined to get a new unit. Would it not have served you to first check out the old card in the new unit. You might have saved some money if it worked or exchanged it for a different card if it did not. In the process you would have pinpointed more accurately which component was at fault. JMTC