Now, when VM tries to start Windows 10 it gets caught in a loop. Originally running Parallels 11 (now Parallels 13) with Yosemite, I accepted Windows update and was surprised by the popup window saying the Mac OS had prevented writing to MBR on Bootcamp partition. Now I seem to be trapped between old and new with no access to either. The virtual Windows machine would interrupt its boot and try to revert to earlier version but never completed that task either. Never had this experience before with any previous 6 versions of Parallels and hope there's a way to fix the Master Boot Record on the bootcamp without completely re-constructing the Windows virtual machine. Similar experiences and suggestions would be greatly appreciated