Import from another catalog

Lightroom version 10.0 / Win 10 Pro v2004

Hi, I'm hoping someone can please help..

I want to get some developed DNG files from my laptop to the mother PC where my master catalog resides.

I've exported the folder of photos from LrC on my laptop as a catalog to my portable HD. I'm now trying to import (merge) this catalog with the Master catalog on my main desktop PC.

When I choose the LRcat file in the 'Import from Catalog' dialog box, I see my photos and folders, but in the 'File Handling' drop down menu there is no option to 'Import To A New Location', the only two options are  'Don't Import' and 'Add New Photos To Catalog Without Moving' (The files are currently sitting on my portable external hard drive) 

I changed the 'Copy to:..' location to the folder in the folder structure on my PC where I want them and attemted to merge, but after what looks like a successful import,  I get an error from LR saying it couldn't import the photos and lists them all. Then thumbnails appear in Library , but the folder hasn't been imported and each photo has an exclamation mark with the info that the original file can't be found. I don't want to import my portable hard drive. Can anyone please tell me where I'm going wrong? Many thanks