If we take 2010 aggregate company unit shipments and add the unit shipments of Seagate to Samsung and Western Digital to HGST, we find the combinations should have resulted in about 41% unit share for Seagate plus Samsung, 48% unit share for Western Digital plus Hitachi GST and 11% unit share for Toshiba.
FORBES: Why Government Regulators Should Approve Hard Disk Drive Company Consolidation