Sharing Custom Categories on 2 computers?

I have a desktop pc (that does all the heavy lifting work) and a laptop for simpler stuff. I have my content library on an external usb hdd that they both share, and DIM and it's manifest folder is on that same drive, so both computers see the same installed/ready to install content.
The one thing that I can't figure out if there's a way to share, are my custom categories. I know I can export the user data and reimport it on the other computer, which I've done a few times, but it would be nice if both machines could just automatically share the same categories. As it is, I'm keeping a list of what I've newly installed on each computer, then manually adding the new products to the appropriate custom categories the next time I'm using the other computer.
I can't seem to find if the custom categories are actually stored somewhere, and there's a file that could be moved onto the external library hdd for both computers to read. I'm guessing it can't be done, but no harm in asking...
Comments
You could put the database on the external drive too - set the ContentCluster folder in Edit>Preferences>CMS Settings.
I just tried that and lost all the categories I had set up, any advice?