Just got my new SanDisk Plus 128GB disk (48mb/s, £39.99 from Amazon UK):
It's been on my surface 3 for a few hours, and so far it has not unmounted, or has not caused any other trouble. It has gone sleep a few times, but every time it came back OK so far. The capacity seems to be the standard 119GB. I am now listening to music from the disk via Groove Music, but it seems to be running smoothly so far....
I definitely need lots more testing to get a feeling about the consistency, but so far it's been OK...
Snow Cat