SMB connection to external Thunderbolt drive not working in MacOS Catalina
To solve this issue you have to do the following:

Open System preferences
Click on Security &amp; Privacy
Click on the Lock in lower left corner to unlock
Click on Full Disk Access
Click on the + button
Navigate to /usr/sbin
Seach for smbd and select it
Click open

Now the SMB daemon should have permission to connect to the external drive.