Or consider option D
You have misunderstood the error message.
Your photos can go on any drive including network drives. The Lightroom catalog file cannot go on a network drive.
Does that work?
To Lightroom, the term "catalog" does not equal the images or picture files Lightroom is editing; it refers to the LR catalog file, a database file maintained by Lightroom. One thing the LR catalog file does is keep track of the location of the files it is editing. iTunes does something similar.
You say you're looking for more room to store your images. (The Lightroom catalog file is much smaller than the sum of the image files.) You can keep Lightroom's system files on a Mac computer and store your images on a separate networked drive. To do this, allow LR to set up the catalog file on the main drive (or if you prefer a separate partition the Mac is using). Then import the images *using Lightroom* to the external drive. If you've already imported some you want to use into Lightroom on your Mac's hard drive, use Lightroom to move them to the networked drive. That way, the LR catalog file knows the new location. The LR Catalog file will lose track of them if you use Finder to move them.
These are both excellent suggestions. I'll try option d, and see where that goes. Thank you very much for your suggestions - Lightroom is an awesome application; previous versions allowed catalogues on networked partitions, so this error message was unexpected.
Correct. If you had an earlier version of LR catalog file and your image files on an external drive, create the LR6 catalog file on a drive on the Mac, and the new catalog file should already know your image files are on the external drive after it updates the catalog file from its earlier version.
previous versions allowed catalogues on networked partitions, so this error message was unexpected
It has never allowed this! You could always have catalogs on external drive (USB, thunderbolt, etc.) but never on networked drives. This has to do with file locking issues with the database code (SQLite) that Lightroom uses that can corrupt the catalog file if the file locking is not correctly implemented. I am not sure how big of a problem this really is but it is the official reason why this is not supported.