Lore

Built in Dr. Soong's own image, first successful, fully functional positronic brain
Emotionally unstable, saw himself as superior to Humans, frightened the colonists
Secretly contacted the Crystalline Entity, was deactivated by Soong & put in storage
2364: Found & reassembled by the Enterprise-D; deactivated & masqueraded as Data;
Tried to destroy the ship but was stopped by Data, who beamed him into space
Drifted in space for nearly two years, rescued by a Pakled trade ship
2366: Summoned to Terlina III by Dr. Soong, Stole Data’s emotion chip;
It caused him to become even more unstable, accidentally mortally wounded Soong
2369: Appointed himself leader of a group of Borg disconnected from the Collective;
Used Data’s emotion chip to lure him to his side, was stopped & deactivated
2400: Memories were downloaded into the Daystrom Android M-5-10