@wila There's a 1.46GB recovery partition at the beginning of the disk, and then a 227.16GB bootable NTFS partition where W7 is. See attached screenshots for details.

PS: these screenshots were from yesterday. I was able to fix the "this volume does not contain a recognized file system" error by 1) marking the boot partition as "active" inside W7's disk manager (strange it wasn't); and 2) deleting the file C:\Boot\BCD and recreating it by running "bootrec /rebuildbcd".

I also ran CHKDSK and made sure the disk has no errors. Still, the problem persists. Today I'll try to make a data copy to a new NVMe disk file (without the partitions and boot) and see if W10 reads it and recognizes it as an "existing installation of Windows" that can be updated. Shooting in the dark now...