Are you facing this issue with all the catalogs or just the one you have on the external drive.
Would you please try to launch Lightroom holding the option try and try to open your catalog as well.
It is just the catalog I have on the drive
For some reason when I held the alt/option key, the catalog opened! Why is that?
Okay although I thought it was fixed, it seems to be doing it again today.
Would you please change the catalog location from your external drive.
Launch your catalog from the new location.
I did that on the first day it was happening, and the catalog opened fine. However, yesterday when it was working again I used a backup I had made and that worked fine.