I've just had a lot of fun with this ...  My MD802 came originally with an 8GB but I bought a 16GB Lexar 633x (95MB/s) and the unit will not even light up with it.  I've formatted the card several times, installed only the system software and it would not boot up (black screen but power light on). I have a Duracell 16GB class 10 (I don't even remember where I got it - I had it in another device) and it's working...  I don't think the speed is so much the issue but rather the power consumption by the card.  I do think the faster cards do require higher power consumption though.

Another interesting thing I've noticed was I had to format the card with a 4kb cluster size.  The default on this 16GB card with FAT32 was 32kb and it seemed to have issues writing and such.  My 8GB card had a 4kb cluster size so I matched up.  I know in the Microsoft world, NT and up had problems with >4kb cluster sizes so maybe the Autel tool experiences similar.

So I can confirm it's working with a 16GB card now but it won't work with all.