Unmountable Boot Volume

guess the problem could also be in the security for the UEFI
you might have to set it to legacy mode if the drive signatures don't match.
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depending on the cause of the error (see diskpart info below)
often a fix will be to boot on a repair image and run

bootrec.exe /fixmbr
bootrec.exe /scanos
bootrec.exe /fixboot
bootrec.exe /RebuildBcd

often it is easier to just apply a fix than trying to figure out the problem.

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boot the system on a repair image, bring up a command prompt and run
diskpart.exe (see below for some commands) what you are looking for is that your drive c is still assigned to your windows drive and the file system type is not marked as RAW

(it is common for intel storage drivers to reassign the drive letters incorrectly.
you can use the help command to see the various functions that diskpart.exe has to fix problems or just show you what is wrong)

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PS C:\WINDOWS\system32> diskpart.exe

Microsoft DiskPart version 10.0.17134.1

Copyright (C) Microsoft Corporation.
On computer: WINGNUT

DISKPART> list disk

Disk ### Status Size Free Dyn Gpt
-------- ------------- ------- ------- --- ---
Disk 0 Online 238 GB 0 B *

DISKPART> select disk 0

Disk 0 is now the selected disk.

DISKPART> list partition

Partition ### Type Size Offset
------------- ---------------- ------- -------
Partition 1 System 500 MB 1024 KB
Partition 2 Reserved 128 MB 501 MB
Partition 3 Primary 237 GB 629 MB
Partition 4 Recovery 450 MB 238 GB

DISKPART> list volume

Volume ### Ltr Label Fs Type Size Status Info
---------- --- ----------- ----- ---------- ------- --------- --------
Volume 0 C NTFS Partition 237 GB Healthy Boot
Volume 1 D NTFS Partition 450 MB Healthy
Volume 2 ESP FAT32 Partition 500 MB Healthy System

DISKPART> exit

Leaving DiskPart...