Is your system a Win 10 upgrade, possibly from Win 7?
There are several threads about boot problems with messages similar to yours. Basically the system is looking for direction as to how to boot your system. The instructions are in the small partition on the drive. So something is corrupted or not where the boot system thinks it is supposed to be. Which is what the link from Regedit32 describes.
I will assume you have not changed anything on your system as far as drives so the Bios would have no reason to be looking at the wrong drive or looking at the wrong partition because another drive was added. If this is an MBR system, if an alternate Active partition was available, it might confuse the system.
If you go through the link and get to the Bootrec command, there are times when that command will not work. Let us know because there is another command you can try to replace the boot files named
Bcdboot.exe C:\Windows
and will create a new BCD Store were there may be none for whatever reason.
But as the link shows you need to be in a Recovery Command Prompt from bootable media in your situation.