If there are multiple drives installed in the computer, then it can create confusion as to which drive has a Bootloader. This scenario usually happens when Master Boot Record is on the wrong drive.

The System Reserved Partition is a special partition in Windows that is invisible to users, including administrators. It is designed such that no drive letter is ...

If done the other way, Windows will just replace the boot manager and ignore the fact that another OS is installed. Perhaps if you were to do a clean install of Windows to the second drive, in UEFI ...