I just purchased a new surface. One of the reasons I went with the 128GB is because I could add that much via the micro SD slot for cheaper than upgrading. So I did. My plans were to put all of my sync folders and dev folders on that card, and use the other for programs. I had to jump through some hoops because Dropbox doesn't allow you to use an SD Card, but I created a junction, and everything is fine. Or I thought so.
Suddenly, OneDrive started crashing on startup with no message. I checked the event logs, and that was not very useful - fault in ntdll.dll. I was about to give up, and just change my plans, when I came across a link that gave a possible solution- change it from exFAT (the default on those cards) to NTFS. So, went through moving everything to my internal, then formatting it... then moving it back... and lo and behold, it started working again.
Just posting this here in case anyone else has a similar problem (and hoping that it stays stable).