solid state drive

i install a new ssd and plan to used it as C; when is plug in by itself it works, but when i plug the other drive the system always choses the old drive to boot, i see the new drive in the main screen, but not in the boot priority one, how can i reassign the drives , right now the system don't let me change the letter on the old partition with the os. in it

thanks for making me look a little deeper, the actual problem was that i needed to choose which hard drive the system was to pull the os from; so thanks for helping

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