Most of what a company backs up each week is the same data it backed up the week before. Data Domain built boxes that noticed. Its appliances stored each repeated chunk only once, shrinking backups by 10 to 30 times. That made it cheap enough to back up to disk instead of tape, which was slow and easy to lose. The Santa Clara company started shipping in 2004 and went public in June 2007.

A Professor's Compression Trick

Princeton computer scientist Kai Li co-founded Data Domain in October 2001 with Ben Zhu and Brian Biles. The company called its method Global Compression. Today the idea goes by a plainer name, deduplication. Data Domain packed it into appliances built from standard parts that plugged into a company's existing backup software. The bet was that IT departments would happily retire their tape libraries if disk cost about the same and restored files far faster.

A Bidding War Ends It

Data Domain priced at $15 a share in June 2007 and listed on Nasdaq as DDUP. The shares rose sharply on the first day. Two years later NetApp agreed to buy the company. EMC jumped in with a higher all-cash offer, and after weeks of rival bids EMC won in July 2009 for about $2.1 billion. Data Domain lived on as EMC's backup line, and later as part of Dell.

Cover: Library of Winchester College, William Bennett. Art Institute of Chicago, public domain. Chosen for a library (one copy of everything).