Unable to backup LR Catalog files upon closing LR
Hi guys,
When I tried to close LR - backing up the LR Cat files in the process, I got the following:

I removed my external drive for these backups and tried to close LR again. The same alert re-appeared and I had no option but to click Continue, but in the end LR closed without the files being backed up. I read their explanations in Learn More which meant nothing to me - too technical.
What's going on and what now? - in plain English, please. Thanks a lot.
Raphael
