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Lightroom cannot find the files because they are not in the location expected by Lightroom. This can happen beacuse you (accidentally?) moved, deleted or renamed the photos or folders outside of Lightroom.
The instructions to fix this are here: Adobe Lightroom - Find moved or missing files and folders
Thank you. I quit and restarted LR and all of the photos associated again
with the external drive except for one collection. I had not altered the
files on the drive and I do not know what could have happened here. Should
I delete the collection in LR and re-import them from the external drive?
Should I delete the collection in LR and re-import them from the external drive?
IMPORTANT LIGHTROOM PRINCIPLE: Never re-import photos. That's NEVER, as in not in the next 20 trillion years, don't do it, don't think it, do not re-import photos.
If some photos are still showing the exclamation point icon as missing, then use the instructions I provided.