Maya won't save as .mb

I had an issue today where I couldn't save my Maya file as a .mb, and faced the following error:

The Problem:

 "File contains unknown nodes or data. To preserve this information, the current file type cannot be changed. //"

This can be caused by having unknown nodes in the scene, usually from an external plugin that you no longer have.

The Fix:

unknown_nodes = cmds.ls(type="unknown")
# Before you delete, check you want to delete these unknown nodes!
cmds.delete(unknown_nodes)

After running this code, you should be able to save as .mb again!