Windows 10 Boot manager removal for dual OS on SSD and HDD

Run > Msconfig, delete entry you don't want to be in the menu.

The way to make it so you can boot from either, switching in bios is to disconnect SSD and reinstall Windows 10 on HDD is to it has its own system reserved or EFI partition rather than sharing one on SSD.

More tricky if you want to keep installed version on HDD i.e. you will need then copy the EFI partition from SSD (or system reserved partition) to HDD before disconnecting SSD. If a system reserver partition, you will need to make it active as well. Minitool Partition Wizard Free is good to copy partition.

Once copied then disconnect SSD and boot from HDD and delete boot entry for SSD from MS config. This is not needed if you clean installed to HDD.

Finally, replace SSD and boot from that and remove boot entry using MS config.

Shrink the OS on the HDD by about 150MB. MiniTool Partition Wizard Free is very good for that. Then open a Command Prompt or PowerShell and run diskpart.

Once in diskpart the commands are:
list disk
select disk # <-replace # with the actual number for the HDD
create part efi
format fs=fat32 quick
assign letter=t
exit

Then in a command prompt (admin) or "run as administrator" command prompt run:

bcdboot C:\Windows /s T: /f all

Note - the drive letter in red needs to be the Windows installation on the HDD. If you are booted into the OS on the HDD, it should be C:. It might be E: or F: or something else if you are booted into the OS on the SSD. Just make sure you do the above commands all from the same OS.

That should make the HDD bootable by itself. When you are done, and it is booting from F12, you can remove the drive letter T: from the EFI partition, and remove the dual boot menu from the SSD by using the msconfig command.