Very odd situation. I have a new iMac and an older Macbook Pro. I have my images and catalog stored on an external HD. When I try to import the images into LR Classic on the iMac I can find the external HD and folder the images are in but LR won't read the images. This is a big pain. What is going on?
On the computer, outside if LrC, etc. Bring up System Preferences. Select Security and Privacy, select Privacy tab, scroll down to Files and Folders, for Lightroom Classic, make sure all media types are checked, probably removable is not.