I've had the same issue with many types of brands from Samsung to Sandisk to Transcend etc. All these cards work with other tablets just fine but on the Surface, unmounts literally within minutes, ejecting then reinserting gets it going temporarily but not to the point where it is reliable opening a file from it. Lo and behold I come across this brand of microsd's http://www.amazon.com/Silicon-Power-MicroSDXC-Class10-SP064GBSTXBU1V20SP/dp/B00TB8XZOK/ref=sr\_1\_1?ie=UTF8&qid=1444258118&sr=8-1&keywords=silicon+power+micro+sd
Not only cheaper than the others but I have not experienced any unmounting since purchasing, going on about 2 mos now. Not a household name but works and is fast. I'm in IT and a co-worker bought USB 3 drives for our group by the same brand. I was impressed at how well they worked and their speed. Figured worth a shot trying their microsd's, if they weren't any good, I could just return them. So far so good and I think it's my answer for the microsd card problem everyone seems to have. I don't know what makes these different from the others but they work. How good they are long term, don't know but for now worth the risk and so far very happy. I'd recommend taking a stab.