It’s not entirely clear what you mean by not showing up on the iMac. Do you mean not showing up in the LR Library? Are the folders visible in Finder? If so you need to import to get the photos managed by LR.
Click the import button and choose “Add” then navigate to a folder on the external drive.
sorry for the confusion. Yes they are not showing up in the LR Library on my IMac and yes they are visible in Finder. So what you are saying is overtime I travel and load images from my laptop to the external hard drive, once I get home and open LR on my IMac I need to import them to my LR Library on it?
Now when you click on the import button and choose "Add", you say navigate to a folder on the external drive, I am confused, do you have to rename the folder and it will appear again on the external HD or do you just click on the folder that already has the photos? Not sure if this makes sense to you or not, I hope so?
I would suggest installing Lightroom on both the iMac and Macbook pro. You can then pick your best shots, edit and add metadata etc. using a separate travel catalog.
I regularly use a travel catalog which I would recommend. I do this on my Macbook Pro using a 128gb SD card but it will also work with an external hard drive. It means when I get home, I can import the travel catalog to Lightroom on the desktop PC to combine it with the master catalog with all my develop settings intact.
Here is a video tutorial which demonstrates the use of a catalog on location.
Yes, for your new photos on the external drive, you need to import them into LR on your home computer so that LR knows about them.
In the import dialog, "add" means add the new images from where they are without copying them nor renaming them. From inside the import dialog you just navigate to the folder containing the new images so that LR knows which images you are talking about.
I do have LRCC installed on both computers, it was just a matter of having my IMac have them in that Library
One option would be to put the LR catalog on the external hard drive with the images. That way you both computers would be using the same catalog.