Today I had to move a Debian based Nessus installation on a physical Machine to a virtual one. After a bit of head scratching I came up with a method. I could not find much on the internet so here is a guide:
1.Connect to the server you want to copy from. I used Filezilla with secure copy
2. Install Nessus on the new machine but don't start the service
3. Ensure the Nessus instance is stopped
3. Copy the entire /opt/nessus directory and overwrite it
4. Start Nessus
5. If you get a error about global.db being corrupted follow the on screen instructions to fix it
6. Release your activation number if applicable in the Nessus Support Portal.
7. Run the following: /opt/nessus/sbin/nessuscli fix --reset
8. Run the following: /opt/nessus/sbin/nessuscli fix --register
activation key here
9. Finally start the Nessus Service
10. Login like normal with the same username and password as you had set previously.
You will find your entire installation will have been copied over and your license restored.