I want to export a folder as a catalog from my iMac to an external hard drive. I will then edit the photos on the external hard drive in Lightroom on my MacBook Air. After I finish editing with the MacBook Air laptop, how do I then re-integrate the photos that were changed in the catalog on the external hard drive back into my master catalog on the iMac?
Once you done with the editing on the exported catalog, you can merge it with the master catalog.
When you go to file menu there's option called " Import from another Catalog".
Refer the related link: Adobe Photoshop Lightroom Help | Import photos into Lightroom
Let us know if that helps.