Have you moved the files or folders or renamed using your computer operating system? If so you will have broken the links in the LR Library. You need to put everything back in the correct location and ensure on Windows that the external drive is connected to the same port to maintain the drive letter.
See this useful link from Ian:
Thank you 99jon!
I ejected the external HD once connected it to a different computer, then back into the original computer, my mac, but it’s very likely, that I plugged it into a different port. How can I find out? I appreciate your help very much!
Are you using Windows or Mac?
On Windows the drive letter and location should display in the Library when you select a thumbnail and hover over file path in the metadata panel. Windows Explorer will show the drive letter for the connected ehd.
On Mac the ehd should be recognized by its name.
I am on a Mac
Are there question mark icons on your folders, or exclamation point icons on your photos, as shown in the link given above by 99jon? If so, then you need to follow the instructions at that link to fix the problem.
Thank you dj_paige for your reply! There are no ? on my folders, but all the photo files inside have the ! icons in the top right corner. I am on a mac OS X Yosemite.
Thank you, dj_paige,
I think part of my LR catalog is corrupted, because I did some rearranging of folders and photos and deleted some copies of photos on one of my ehd. Now those photos from that ehd show the ! icon in the right upper corner.
What would I have to do to clean this up. Locating the photos with the finder didn’t help.
Thank you for your help!
This is not a corrupted catalog. If it was corrupted, the catalog would not open.
It is a direct result of you rearranging the folders on your hard disk.
The instructions at the link given will solve the problem. Please give it a try.