A faulty connection between the two biggest operators of U.S. stock exchanges brought half of the world’s largest equity market to a standstill, making this the second time this week that U.S. trading was shaken by a computer malfunction. Connectivity was disrupted between NYSE Arca and the data processing subsidiary of Nasdaq Stock Market, which led Nasdaq to freeze thousands of stocks from Apple to Facebook that trade on about 50 markets from Kansas to New Jersey - for more than three hours. Just three days ago, Goldman Sachs, which made $5.8 billion from stock trading in 2012, flooded options markets with unintentional orders. The disruption is the latest to expose unreliability in electronic markets just as individual investors who withdrew from stocks after the global economic crisis have shown signs of embracing equities. Failures are increasing as global markets get more fragmented.