Offline thumbnails not showing

Greetings ~ I'm new to C1 and have a question about offline images not displaying in C1 Pro. When I moved to C1 from LR I imported my LR archive onto an external disk and connected it to C1 as a catalog. When the external drive containing this archive is not connected the images nonetheless display, albeit with the question mark or "offline" notice. This is as I expected. 

I have, with the start of the new year attached a new external drive to my computer for this year's captures. I have downloaded new images into C1 and directed them to be stored on the new external drive as a new catalog. The images display fine and can be edited normally. However when I unmount the new external drive containing the catalog the offline thumbnails do not appear as I expected. The catalog cannot be opened. I can only see/work with them when the external drive is mounted. What did I do wrong? And how can I rectify it? I'm on a Mac, 14.1.2. Although I don't think it should make a difference, this time when I imported the images from the SD card I also backed them up to a Dropbox file I created for that purpose. (I mention this to provide all the information.)

Thanks for your help in advance. It is much appreciated. 

Doug Bruns