Do you need/want to "move these files from your backup hard drive to another permanent drive?
Or if your back up hard drive is attached and you want to leave the files where they are (just add them to the Library), you can go to Drives & Devices and use the "Explorer" to located the drive/folder/files and reimport (find the file, right click on it, select import into Library). Note that "Explorer" needs to be turned on in Tools>Options>General>Features.