Hi Richard, Let's take the second question first, since that's the easy one. Perhaps miscommunication on one side or the other... Lightroom mobile can only sync with one Catalog (not one Collection). If you attempt to sync a different catalog, Lightroom will warn you and then if you proceed, you'll replace all synced images with the ones form the new catalog. Now as to the first query about collections. You can create collections on Lightroom mobile as well as on the desktop. Any synced photo from any collection will appear in the collection called "Lightroom Photos" on your mobile device, but called "All synced photos" in the Catalog panel of Lightroom desktop. If you have "Auto-add" enabled in Lightroom Mobile settings, but don't have any specific collection enabled with Auto-add, any new images detected by Lightroom mobile will be added to the "Lightroom Photos" collection and synced to your desktop, but will not appear in any of the other collections. I find this to be a bit troublesome, as it's hard to track down synced photos that aren't in a specific collection (so I always keep a special general purpose catalog called "On the Go" with Auto-Add enabled, so I know where to find those photos). I hope this helps - if any of this isn't quite clear, please do let me know. Mike ... View more

Hi, I believe you've confused "Smart Collection" with "Smart Preview." In order to work with your images offline (hard drive disconnected) you'll need to build Smart Previews. You can do this by selecting the images (when the images are online and connected) and choosing from the menu: Library > Previews > Build Smart Previews A few things to note: 1. You don't have to build smart previews for every image in your catalog. If you think you're going to want to work mostly with the previous month's images, just build those smart previews. If you'll be working with a portfolio you've saved in a collection, just build those smart previews. If you build smart previews for 12,800 images, it will take a lot of time and space that may not be completely necessary. 2. Videos cannot have smart previews. If your catalog includes any videos, you won't be able to work with them when the external drive is offline. I hope this helps! Mike ... View more

Hi Raphael, Lightroom stores backups of the catalog in a zip compressed format to save space. If you need to recover from a backup, you'll need to unzip the archive to get the .lrcat file backup you need. Because your catalog file file is over 4Gb, your native MacOS tools can't extract it. You'll need to use a 3rd party product to open these archives in the future (a utility such as 7zip or Stuffit Expander). This dialog is just an informational warning. It's new, because in earlier releases, if your catalog exceeded 4Gb, LR didn't compress it. Mike ... View more