Yesterday I learned a reliable way to merge a temporary Lightroom catalog into my main catalog.
While we were traveling, I considered bringing my external drive with my photos but didn't want to risk damaging it or losing the images in it. Instead, I brought only a netbook with me. I created a temporary catalog for the trip and imported photos into it the way as I usually do. I didn't want to deal with having to carry around another external drive (which could also get damaged), so I just stored the images into the netbook's local hard drive.
When we came back home, I found instructions for merging catalogs. The instructions recommended simply copying the temporary catalog and the photos in them to an external drive, connecting the external drive to my main computer, then importing the catalog into Lightroom. I did this but it didn't work for me - after importing the catalog, Lightroom couldn't find the photos. In retrospect, I think it's because when I copied the netbook catalog and netbook photos into the external drive, their locations changed. It was very tedious to have to locate the missing photos.
Instead of slogging through to locate the photos, I decided to start over again and had to restore the backup catalog on my computer. This time I used a different process:
1. On the external drive (for transferring the photos), I created a temporary directory.
2. On my netbook, I launched Lightroom, went to the Library module, and selected all the photos. I then clicked on File .. Export as Catalog. I checked the box for exporting the "digital negatives" (meaning the original unedited files) and the existing previews (I exported the previews to save time later). I saved the exported catalog into the temporary directory in the external drive. The temporary directory now contained not only the catalog and photos (with metadata), but the exported catalog knew the correct location of the photos within the external drive.
3. I connected the external drive to my main computer, launched Lightroom, then clicked on Import from Catalog.
4. In the destination folder, I chose the folder where I would normally store the photos.
5. After making sure all photos were imported correctly, I deleted the temporary drive on the external drive, and the photos and catalog on the netbook.
This process successfully merged the temporary photos (including all edits) with my main catalog. The folder structure of the temporary photos was preserved except that they were now subdirectories under the folder to which I imported them.
In my view, the extra step to Export as Catalog is worth the peace of mind of being able to locate the photos later. Plus, I don't think exporting the catalog takes significantly more time than copying the photos and catalog manually.