Just to share with others:
This issue also happened with W8 and hasn't been changed:
When installing W10 on a hard drive where another operating system is present (Dual boot or swapping OS by changing drives.) W10 corrupts any other windows OS it finds W7 for example. Typically you get a message while running W10 which says "reboot required to fix HD"
Switch off the Fast start up feature under Power option in W10. This stops W10 from running a chkdsk and finding disk faults which don't actually exist, or changing the boot options. If you don't, then W10 will make changes and W7 recovery options will not work because the drive can't be read. (Image recovery only)
Took me severel hours to "remember" this fault on Windows 8 which has still not been corrected and is much documented; if you know where to search.
(This is NOT a case of upgrade problems to W10. This is W10 reading another drive containing a windows OS.)