Have you moved your images to the external drive using Explorer/Finder?
Can you please post a screenshot of your folder panel?
I simply dragged them out off my desk top on to my HD
If you want Lightroom to keep track of your images then you have to use Lightroom to move them. Even though Explorer/Finder is called by Lightroom, you are using the operating system to move the files. In the library module, the folder that contained those images should also display "?" Which indicates that Lightroom no longer knows where those images are located. Right click on that folder and you will be presented with options that will enable you to point Lightroom to the appropriate folder on your HD.
Whenever you have to make changes or move the pictures make sure you use the lightroom for this.
However when you pulled the files from Desktop to Hard Disk the catalog is trying to find the files at the default locations.
The ? mark is the sign of this. Hence please follow the links below which will be helpful.
This is a screen shot I hope you can see. I attached my HD and did the right click. I didn't know I had to steer LR exactly to the folder on my HD. I thought it sees it. In any case I think I showed it as seen on the left but there are still "question marks (??)" Please help and I appreciate your earlier comments. Penni
Please follow the steps given above....
At this time the LR is picking up the thumbnail previews from the cache however the original files are missing which is shown by question mark.