I have been using Lightroom Classic for a few years now. My laptop is out of room so I purchased a 1TB external drive. I have been saving all my SD cards since 2010 and decided to load all those photos into Lightroom but instead of going to my C: drive I instructed the imports to go to my external drive. All importing went well. They're on the external drive and they're kindof in Lightroom. In Lightroom Library they all have an explanation point in the upper right corner of each photo. In Develop they each say the folder could not be found. WHY?
Because you mention "C drive" I assume you are using Windows. What often happens on Windows machines is that the drive letter of the external drive gets changed, for example because you added some other peripheral that was recognised just a little faster on startup and so was assigned the lower letter that your external drive used to have. The result is that Lightroom is now looking for the images on the wrong 'drive'. It looks for images on the D drive, but your external drive is now the E drive.