I once noticed on one computer that if you disconnect drive D, Windows 10 does not start; as it turned out, during the installation of Windows, the bootloader was not installed on drive C. Therefore, I connected drive D back to the computer, started Windows and wrote the bootloader to the disk C.
First of all, open “Control Panel” – “System and Security” – “Create and format hard disk partitions” or press Win+R and run the command:
I looked at drive D, at the beginning there was a 100M partition with a bootloader, so I needed to create a similar one on drive C (not necessarily at the beginning of the disk). To do this, I clicked on the C drive and selected “Shrink volume”, indicated 102M, as a result, a 100M partition was created, which was formatted into the FAT32 file system and indicated the first free drive letter F:.
When the partition for the bootloader has been created, all that remains is to copy it; to do this, I launched the command line (cmd) as Administrator and ran the command:
bcdboot C:\Windows /s F: /f UEFI Boot files successfully created.
I turned off the computer, disconnected drive D, after which the computer successfully started from drive C.
It remains to remove the drive letter on the partition with the bootloader so that it is not visible in Explorer. To do this, I opened diskmgmt.msc again, right-clicked on this partition and selected “Change drive letter or disk path…”.
By the way, I then formatted drive D, and to remove the boot partition at the beginning of the disk and the recovery partition at the end, you can use various free utilities, for example, I used AOMEI Partition Assistant.