I have lost photos from an external hard drive and want to export them from Lightroom again. When I look at every collection it is offline or missing. I know that - because I can't find them either. How can I export the photos in the collection again? (without importing them and editing a second time)
To do an export, LR needs access to the original photo files. You probably see a question mark on several folders in the library and/or exclamation marks on photos.
You need to connect the drive where the photos are located or if they are moved reconnect the folders/photos to its new locations.