Reproduction steps:
1. Create a new project in an External Storage Device (HDD/SSD)
2. Close the project and delete the Library folder in the root of the project
2. IMPORTANT: read notes provided before doing the next step
4. Try opening the project

Unexpected behaviour: Unity throws a Fatal Error "attempt to write a readonly database"

Reproducible with: 2018.4.22f1, 2019.3.14f1, 2020.1.0b8, 2020.2.0a12

Hub version 2.3.1 (beta)
Devices used for testing: iMac Catalina 10.15.3 + Samsung Portable SSD T5 ExFAT (1 Terabyte)

Not reproducible with MacBook Pro 15 2015, macOS10.14.6 External HDD (not SSD) formated as Mac OS Extended (Journaled)

Notes:
1. After pressing quit on error window you might receive a request for Developer Tools Access (case 1248630). Do not enter your password. Force Quit Unity using Activity Monitor and then proceed to close all Unity windows (you will need to restart your PC if you fail to do this as it will become impossible to shut down Unity)
2. Switching the project to another version does not make the project suited for reproduction. It seems some files must be missing.
3. I am not sure what exact changes need to be taken to make the project suited for reproducing this. Some existing projects just won't open even if their Libraries have recently been re-built.
4. Video attached in edits (2020-05-20 15-26-44.mp4)