what motherboard do you have?
simplest way to copy info might be to boot from installer but we obviously need to get past that message.
boot from installer
on screen after languages, choose repair this pc, not install.
choose troubleshoot
choose advanced
choose command prompt
type notepad and press enter
in notepad, select file>open
Use file explorer to copy any files you need to save to USB or hdd
then I would really start from scratch, you don't want 2 installs on 1 pc, it is just a waste of space.
I wonder if win 10 created a new boot partition or used the one from old install - this is why starting again is likely best choice.
I would copy everything off the old install onto new.
Turn off PC, unplug power, and take the new install out of PC
reinstall win 10 on normal drive
once PC boots from old drive, turn it off, put old hdd back in and make sure its not added to boot menu in bios
boot from normal drive, copy info off new install onto the right one
delete the new install.