Mac Users running Lightroom 6.8 or CC 2015.8: have you gone to back up your Lightroom catalog (like I tell you to!), and encountered this message that your catalog is larger than 4 GB, and that you’ll need a third party utility to extract it?
First, as of right now there is a bug – all Mac users are getting this message and most of you don’t have catalogs this large. (Update: this bug was fixed with 6.9 / CC 2015.9 – now you’ll only see the message if your catalog is indeed over 4 GB.) You can check the size of your catalog by going to Lightroom>Catalog Settings – it displays on the General tab:
Back to the 4 GB warning dialog: regardless of how big your catalog is, consider checking “Don’t Show Again”, and then hit “Continue” and back up. (Remember, your Lightroom catalog contains all the work you have ever done on all your photos – so if it becomes damaged and you don’t have a backup, you will most likely lose all that work!)
Here’s the scoop: with Lightroom 6 and CC 2015, Adobe started compressing catalog backups into zip files to make them smaller, and to speed up the backup process. However, they didn’t do this for catalogs over 4 GB on Mac, since the default macOS unzip utility is unable to unzip files larger than 4 GB. Starting with Lightroom 6.8 / CC 2015.8, Lightroom now will compress these large 4+ GB catalogs, but if you someday encounter a catalog corruption and need to restore from a backup, you’ll need to use a free third party utility such as 7-zip or StuffItExpander to extract it from the zip file. If in doubt, just double-click on the zip file – if it doesn’t open or says that it is corrupted, you’ll need one of these utilities. However, if you’re lucky and never have a catalog corruption issue, you’ll never need to do this!