The drive is external and always connected so it should't cause a "not found" condition. I finally managed to "forget" the catalog but it took over 10 minutes of fumbling around to get to the dialog that offers the option. Regarding whether I deleted the catalog from within the software or externally through Finder, it was all within the program. I managed to have it happen again when trying to set up a new catalog which is a quirky process. If I open the Catalog Management dialog, select "New Catalog", Browse to the folder containing the photos, the set the name to the desired catalog name, scanning to the new catalog fails because the new catalog is within the folder that is to be scanned (default location, from the dialog). I got it to work by manually choosing the catalog folder to be excluded.
The process of creating a catalog shouldn't require so much manual intervention. Logically, creating and populating a new catalog should all be managed in one dialog where the user gives a name for the new catalog and identifies the source folder. A scan button should be present within the dialog as the next step and the scan should proceed without error because the software will be smart enough to automatically exclude the new folder created in the previous step. I've worked in the software industry for decades (primarily in software testing and user experience) so I'm not a newcomer to databases and I can't help but think that the database side of Photo Mechanic Plus is still too finicky and lacks sufficient documentation to properly use it.
I was hoping that it could become a reliable and logical DAM that could be an improvement over the unreliable DAM that Capture One uses. It's not there yet but I'm rooting for you. Your tech support here is great but I can't help but get the sense that the developers are the same people who test the software. I will periodically check back because it does look like you are continually improving your application and care about your customers.
In the mean time, I will use the standard Photo Mechanic 6, because, even though it's quirky, unconventional and at times confusing in its user experience, it delivers very useful functionality.