I divide my time between Austria and Spain. My main Lightroom setup is in Austria. This consists of a Mac, a SSD external drive with the LR catalogs and two external drives with the images, one with the main images and one that's copied from that main drive every night. All of this is backed up by Backblaze constantly to the cloud.
In Spain, I use my MacBook Air and the SSD external drive with the catalogs. The idea was that I then took my main image drive to Spain and do a backup there to another external drive. Again, everything backed up to Backblaze.
But I screwed up. I brought the correct SSD drive with the catalogs, so that's fine. But I brought the backup image drive and NOT the main image drive. So, I don't have the images here in the folder that LR synced with in Austria. I do have smart previews and I do have copies of the images, but they're not in the folders as recognized by my catalog.
This wouldn't be much of an issue were it not that I'm post-processing images from last year for submission to a stock agency. So, images get tagged, edited and (for some) deleted. I'm doing this on a separate catalog covering about one week worth of images.
I don't know how I merge these new images with the existing ones when I'm back in Vienna and have access to the correct external drive again (the one with the tens of thousands of 'correct' images). Is there a procedure to merge these two catalogs while preserving the work I'm doing now on those images?
My first thought centers around the backup drive you brought with you. A backup drive in my arena means duplicate ie same as the main drive that has the same name and folder structure. If this isn't the case for you, can you rename the drive and folder structure on it so it appears the same as the main drive? Otherwise when you open up LR I suspect it shows in the left panel ?-marks for the location of your photos. You should be able to right click on the folder in question and then point it to where the photos reside on the backup. Of course this will require some jockeying when you return to Vienna to repoint LR back to the main drive but prior to doing that you'll need to ensure what ever was done to actual files such as adding some or deleting others on the backup drive gets duplicated to your main drive.
Since you indicated you are using smart previews LR that may allow a simpler solution but I'll need to think about it a bit more. Hopefully someone here on FM that uses smart prviews will chime in with a solution.
If I understand correctly, you shouldn't do anything with the catalogues themselves. As long as you don't rename any files, you can save all the LR edits to .xmp files while in Austria, and then copy those .xmp files into their appropriate folders on your main drive when you get back to Vienna.
I haven't done this in a long time but I assume that when you have your min drive and the .xmp files correctly placed you should be able to synchronize the folders to update all the metadata. The only problem I can see is you can only choose one set of metadata and not merge edits made previously with keywords and why not assigned while away.
