FWIW, no problems running latest auto-downloaded XBM (version 2.2.849 per my About page) on an RT running 8.1, using both the C drive and a 64gb Sandisk card.
This problem is specific to Windows Phones running the Windows Phone Developer Preview 8.1 OS. I don't know of anyone having this problem on other devices. I don't want any confusion. Thank you though.