Exporting photos from internal drive to external drive Lightroom Classic

You can do it either way, from within LrC, or using the operatings system and re-connect.

It sounds as if you did something wrong, and its hard to say exactly what. Unless you can describe step-by-step what you did, in detail, its hard to reconstruct and pinpoint the error.

The method I recommend is to use your operating system to do the move, and then reconnect in LrC. The instructions are in this document (scroll down to "Part 2 — Updating Folder Location").

You did not do anything wrong. Somehow the thumbnails need to be updated, but because the external drive is disconnected Lightroom can't do that. Lightroom has a new function that is it automatically discards previews that are no longer valid, that's why you see only empty boxes. Connect the external drive again so Lightroom can rebuild the thumbnails. You can force Lightroom to do this by selecting the photos and then choosing Library - Previews - Build Standard-sized Previews. Then try again and it should work.

Edit**. Also very confused as to whether the best practice is to do this via the folders panel in Lightroom or mac finder?  I was under the impression from vids from Scott Kelby, Terry White, etc...that the proper way to do it is via the folders panel.  I have seen a few other posts in here saying it's better to do it outside of Lightroom.  Very confusing.