I have a late 2013 version 27" iMac with a 3.4GHz Intel Core i5, 8GB 1600MHz DDR3 memory, and 1TB HD and running OS X El Capitan V10.11.6. When the iMac was purchased in 2015, LR6 standalone was installed. Four days ago, it seemed to have a catalog corruption so uninstalled LR and then reinstalled (there was an issue doing so but LR Tech Support eventually resolved so have LR6.8 in place). All photos are stored on a 3GB Western Digital My Book external hard drive with the catalog saved on the iMac internal drive. Each time I Quit Lightroom there is a large Problem Report popup indicating that LR quit unexpectedly - how could it when I selected Lightroom/Quit Lightroom?
In addition, I have set Backups for each time Lightroom is exited and I receive a popup indicating the catalog size is more than 4GB and a 3rd party extractor is required. The size of the file Lightroom Catalog.lrcat per Mac Finder is 510.3MB so why is there this popup?
The warning message is the result of a bug introduced in the 6.8/2015.8 version of Lightroom. The message is produced no matter what size of catalogue you have. That at least, you can ignore.
The message about LR quitting unexpectedly is strange. Does LR work properly otherwise?
As near as I can tell, LR seems to work OK despite the "Quit Unexpectedly" message but getting such a message is a bit concerning.
Please update Lightroom to 6.9/2015.9 version which includes the fix for Lightroom: Erroneous warning message on Catalog Backup | Photoshop Family Customer Community