I am waiting on my 906BT device to arrive still, so I have not looked at documentation that comes with it, but I thought I read on some that maybe 32GB was the limit on card size.  Maybe I mis-red.  Android (The base OS) can also be picky about the SD size and format.  You may want to try a few different ones (FAT, FAT32, EXT, NTFS).

Have you sorted out this issue? I am going through the same.
Please advise.

  They seem to only recognise sandisk 32gb or maybe 64gb no bigger or will not work with bigger tf card as i understand it. Hope this helps 

 You need to use a sandisk 32GB class 10 micro SD card. BUT if you do this it will move some of your vehicles to the SD card and you'll have to leave it in there to do those vehicles. I use my slot to back up my SCANS folder EVERYTIME before I do and update. I do this for 2 reasons, one; incase the update craps out and two; incase I lose functionality; for example if say Autel removes functions from updates I still have and can put back a previous version of the scans folder. If I were you I'd delete the vehicles you will never work on to clear up space. Example, Maserati, I wouldn't touch one of those with a (as the Grinch says) 39 and a half foot pole, so I don't waste space on my scan tool with that program. Keep in mind though, You can still back up the scans folder to a USB thumb drive if you use the SD slot, you'll just have to copy the scans folder from both the tool and the SD card. Good Luck !

 Fat 32 SanDisk ultra 32gb work's  on my ms906bt no problem