I made a copy of my 2019.2.8 project and imported it into Unity 2019.3 0f1
I did NOT choose to upgrade to asset importer 2
At the end of import I received a Fatal failure:
Opening file /Users/xxx/Desktop/Projects/Projects/xxx/Unity/Unity Project/Library/ScriptAssemblies/BuiltinAssemblies.stamp: No such file or directory
I clicked try again but it kept looping to the same error.
I chose force quit, the other option. I noticed the project folder itself was missing from my hard drive and from the Unity Hub list.
After a few minutes it reappeared. Ive never seen anything like this happen before.
Im on Mac 10.14.6