Re: Need assistance recovering disk space.
EMC has service offering for EX to S1 migration called 'In-Place Migration' or IPM. That is your best bet for getting EX out of the environment and moving everything under SourceOne control.
My company (Interlock Technology) offers an EMC-certified service for quickly moving SourceOne data from Centera to a CIFS target such as VNX or Isilon while retaining compliance attributes and providing a full audit trail. More information at Financial Services – Email Archive Migration | Interlock Technology.
p.s. @Holger Thanks for the update!