came across this today on the Technet forums so wanted to blog it. Imagine the scenario, someone accidentally deletes the All Unknown Computers collection in CM12. As a result you try to recreate it using manual means but the newly created Collection never populates. Long story short, to create the Unknown Computers collection again automagically do as follows (Thanks Kerwin):-
To recreate the collections, change the registry value of CreatedUnknownDDR under SOFTWARE\Microsoft\SMS\COMPONENTS\SMS_DISCOVERY_DATA_MANAGER on your primary site server (not the CAS). Set the value to 0.
Then, re-start the SMS_EXECUTIVE service. When the Discovery Data Manager (DDM) component starts, it will think that the Unknown System records have not been created, and will recreate them.