Any question answered...

Following this guide; https://www.google.com/amp/s/www.technipages.com/windows-10-fix-unmountable-boot-volume-error/amp

I retried Chkdsk from the HD without using a CD or USB with Win10 on it.

Stages 1-3 seemed to work. But then at the end of stage 4, the computer said no.

Or more specifically:

Stage 4: looking for bad clusters in user file data ...
Windows replaced bad clusters in file 3EAED
of name \Windows\INFUSE~1\Packages\MI63A3~1.0_X\Assets\SECOND~1\Place\LargeTile.scale-200.png
Windows replaced bad clusters in file 40C5A
of name \Windows\WinSxS\AM7AC6~1.15_\xrWPdeft.xst.
  510704 files processed.
File data verification completed.
An unspecified error occurred (75736e6a726e6c2e 4fa).
Failed to transfer logged messages to the event log with status 50.

X:\windows\system32>