Windows 10: UNMOUNTABLE BOOT DRIVE after Windows Update rebooted PC

Hello,

I am a Linux software developer which is not to say I'm wholly unknowledgeable about Windows but I haven't done much "desktop support" since Windows XP. Someone in my household has a Windows 10 PC which they left on Tuesday night and Wednesday morning it had the following blue screen message: "Automatic repair couldn't repair your PC". I'm fairly confident that Windows Update restarted the PC over night. If I click the "Exit and continue to Windows 10" button, I will get a blue screen with the error "UNMOUNTABLE BOOT DRIVE" before it automatically reboots and tries the automatic startup repair.

I've booted the PC with a live Ubuntu disc and Linux has no problem mounting the partitions. I've examined the drive contents and while I don't really know what to look for, there seem to be all the normal folders (Windows, Program Files, etc) and even has the registry files in WINDOWS/System32/config and a backup under RegBack. So I can conclude that the error message "UNMOUNTABLE BOOT DRIVE" is not accurate.

I have also used the command prompt you can get to after the automatic startup repair fails and done all of the things that are often suggested including:
chkdsk /f d:
bootrec.exe /fixboot
bootrec.exe /fixmbr
bootrec.exe /rebuildbcd
sfc /SCANNOW

The chkdsk passes without a single issue reported. The bootrec commands are kind of a ludicrous suggestion since the PC is booting to the recovery menu which means the MBR and boot loader are functioning (and the PC is indeed MBR, not UEFI). The system file checker is the most interesting one as it does 100% of the Verification phase and then immediately fails with the error "Windows Resource Protection could not perform the requested operation".

I made an image of the PC and have reproduced all these issues in a virtual machine so I know there are no hardware issues preventing me from fixing the issue. It's not a big deal for me to reinstall Windows on the PC but I have become obsessed with figuring out this problem of a totally mountable UNMOUNTABLE BOOT DRIVE.

If anyone has further suggestions or knows where there are logs I can look at for more clues to the problem, please respond!

Thank you for your time,
Chris Showers

:)