I had moved the contents of the "Pictures" folder from my Apple Mac Book Pro to the external drive, in order to make more space on the internal hard drive.
Unfortunately, all my Light Room images are now "offline or missing". Therefore, I moved the "Light Room folder" in "Pictures" on the external drive, back to the internal drive but this folder only contains the thumb nails - the original DNG's are still offline or missing.
Obviously, I can move the entire contents of "Pictures" from the external hard drive, back to the internal hard drive and hope that everything lines up again, but I suspect that there might be a better way: using Light Room to re-connect with the missing images somewhere on the external drive.
Would greatly appreciate advice on finding these missing images/repair the damage.