RE: LUNs unknown group won't come online / rebuild Dell CX4-120
Since your drive that you are inserting is not starting the rebuild what you can do is to try to set it as a global hotspare and then see if it starts the rebuild. If it does start the rebuild then let it go & see if it finishes. If the rebuild doesn’t start then what you will need to do is to either get with a data recovery company to see if they can recover the data or you will need to destroy the LUN and recreate it & restore from backup.
Please let us know if you have any other questions.