As NeoSmart explained, I was copying critical boot files from their storage place on a Windows system partition to the boot partition. To give the FAT32 partition a drive letter, I continued in diskpart as follows: select volume 3 > list vol (to see the asterisk confirming that Volume 3 was selected) > assign letter F (where F was not assigned to any of the other listed volumes) > list vol once again to confirm > exit > bcdboot C:\Windows -s F: where, again, C: was the Windows system drive and F: was the FAT32 partition. In my case, that was Volume 3, and it was hidden. SuperUser essentially suggested using diskpart > list volume > look for the FAT32 partition.The ACTIVE command can only be used on fixed MBR disks.” Well, of course, it was GPT, not MBR. Instead, it said, “The selected disk is not a fixed MBR disk. That was supposed to make the Windows partition active. To fix the zero-installations problem, Windows Club suggested diskpart > list vol > notice the number of the volume containing the Windows installation (in my case, volume 1) > sel vol # (in this case, as just indicated, that would be sel vol 1) > active.They were right – that, too, avoided the “Access is denied” error – but, once again, it found zero Windows installations. I downloaded the former, used Rufus to create the bootable USB, and booted with it. Before I found that bootsect suggestion, a SuperUser answer said those bootrec commands might work if I was booting an installer for Windows 8 instead of Windows 10.That fixed the “Access is denied” error, but the last two commands returned “Total identified Windows installations: 0.” For me, the fixboot command yielded “Access is denied.” Later, I saw a SuperUser suggestion to run bootsect /nt60 sys and then retry the fixboot command. Separately, Wondershare said I could try sequentially running bootrec /fixmbr > bootrec /fixboot > bootrec /scanos > bootrec /rebuildbcd.In my case, that produced an error: “The system cannot find the path specified.” If it had worked, Wondershare would have had me follow it with bootrec /fixmbr and bootrec /fixboot. Another Command Prompt option: bootrec /rebuildbcd > Yes, add installation to boot list.The promised SrtTrail.txt file was not there. I typed exit to get out of Diskpart > C: > cd \Windows\System32\Logfiles\Srt\ > dir. This told me that my restored PROGRAMS (i.e., Win10 drive C) partition had been temporarily (and appropriately) assigned to drive C. To access that, I clicked Advanced Options > Troubleshoot > Command Prompt > diskpart > list volume. From that point, Windows Club said that Startup Repair would produce a log file that might be informative.That failed: “Startup Repair couldn’t repair your PC.”
One option, at this point, was to choose Startup Repair > Windows 10.