Lightroom only syncs one catalog with Mobile Devices. As you have a catalog on your Laptop and iMac you naturally have two catalogs.
When you turn on sync in the new blank catalog you should get a message like this:
Its not the Catalogs that are synced, its "a" catalog that is synced to Mobile devices.
I think I understand now: Lightroom won't let allow me to view the Mobile catalog from my laptop as if it's a mobile device, correct? I was expecting it to work like iCloud or any other cloud-based service (i.e. be able to see my server-based files from any device) but it sounds like that's not the case.
Glad I signed up for that Lightroom class next month!
Just because Laptops are mobile, that in itself does not classify them as Mobile Devices. You can see the contents of specific synced collections from ONE catalog on Mobile Devices such as iPhone, Android devices and iPads.
That catalog can reside on a Laptop OR desktop machine. Technically there is no "Mobile" catalog.
Thanks. I look forward to Lightroom changing this arbitrary limitation this if they want me to continue to be a paying customer!