One way out of this situation, Mike, which works for me, is to reinstall windows 8.1 from your image, if you have one. The problem is, I think, coming from the boot manager. After I had put my image back, including the MBR, iI was once again able to read all my files on th external. I did, naturally, have to do a bit of work with easybcd to get the Tech Preview back on the disk manager.