I am happy to know you are getting it sorted and understanding.
how to bring the second copies back to the iMac HD.
The recommended method goes like this-
1. In the Lr folder panel, Make a note of the folder name * on the external drive that you want on the internal HD
2. Exit from Lightroom and use the operating system (Finder/File Explorer) to COPY that whole folder of images from the 'external' to the 'internal' within a suitable location for photos. (Hint: do not choose the desktop)
3. Open Lightroom, select this same folder in the Folder panel of the library view. (as it appears on the external HD)
4. R-CLick the folder and choose "Update folder location..."
5. A Finder window opens, navigate to the new folder location in the internal HD.
6. Click [Select Folder]
7. When Lr now is looking at the images on the internal HD successfully, you can use the OS to later delete the folder on the external HD
(and of necessity you do have other photo backups somewhere else- right?)
* If you have a hierarchy of folders you want to move, you can select the topmost folder and copy it complete with sub-folders, and use the method above.
For the "Second Copies" folders -you should consider them a temporary backup only. They can be deleted after you are sure you have other more permanent backups done that will 'mirror' or match your internal-HD folder structure and photos. (which Lr does not do!)
Also for the next import- Check the Destination panel.!!