Windows 10: Dual Boot Win 7 Ult/Win 10 Pro 1903 x64 - Win10 Startup Failure

I have EasyBCD installed on my C: Win 7 drive and up until today I could freely boot between that and O: Win 10.
Suddenly I get a black failure screen telling my it can't find the Win 10 boot file. I've tried booting a Win 10 Pro 1903 x64 disk and doing Advanced Options, Startup Repair and it fails.
I am currently doing a chkddsk O: /r and will do an sfc later.
Meanwhile I tried reinstalling EasyBCD and letting it auto-configure and will check how that works out when my file checks are finished.

A question: will this work or do I have to change that first drive letter to C:?

:)

 

Win 10 Pro Free Activation with Dual Boot Win 7 Pro

I have Win 7 Pro installed

I have installed Win10 Pro from ISO to separate partition

Dual boot is working fine

If I use my Win 7 Pro Activation Key to activate Win 10 Pro I have the following questions:

Will Win10 Activate for free?

Wil Win 7 continue to be usable?

 

Dual boot licensing, Win 10 x64 + Win 10 x64

I would like to install two instances of Win 10 Pro x64 on one computer and run dual boot. How many licenses are required?

 

Dual-boot Win10/7 or go back to Win 7

This^^

I only use, or recommend, dual boot if a new/separate HD is out of the question. Way less hassle and potential problems.