When you run the File > Export as Catalog command, there's an option for Include Negatives. Ensure this isn't selected.
However, fragmenting control of your picture collection is a recipe for confusion and I would advise you not to proceed. With your folder structure, which makes good sense, you are going to find that on each drive one folder will be controlled by catalogue Subject A, the next by catalogue B, in another folder some pictures will be in catalogue A and some in catalogue C because you shot a mixture on one day, and maybe some pictures will get in both catalogues with different adjustments and metadata (guess which is "right"). Use metadata to categorise your pictures, not breaking up your catalogue.
Thanks, John. I figured it was something simple I was looking right past.
I've been an avid LR user since v2 was in beta. For the entire time I have had a single catalog for the very reasons you've listed. My decision to break them apart has to do with the subject matter being 3 distinct business ventures and an attempt to collaborate with another individual in a very controlled environment (duplicate workstations setups synchronizing files over a NAS where we know exactly what we're each working on - yet to be proven, but determined to make it work).
The use of 3 drives came out of hardware constraints and a given file will only ever be found on 1 of the 3 drives. I use a Microsoft Surface Pro and love what it's done for my workflow, but the original version had a max of a 128GB drive. I've made the best of it by altering my workflow though to use:
C: (128 GB) - Images are offloaded to this SSD for fast access in making selections and processing. Once completed they are moved to D:
D: (64 GB onboard microSD) - All processed images for recent projects that I need to be sure to have with me at all times. Once they can be archived they are moved to G:
G: (2TB external drive) - All processed and archived images that I need access to, but don't need to carry with me.
All 3 sources are backed up to network storage daily.
LR makes the process work because I can simply drag folders from one drive to another within LR and the catalog automatically gets the database updates to always know where the files are located... even if I don't. Someday we'll upgrade to the more current Surface Pro with greater storage capacity, but for now this works.
Thanks again for the feedback. You got me to the exact place I needed to go.