BAGRAM, Afghanistan — A computer drive sold openly Wednesday at a bazaar outside the U.S. air base here holds what appears to be a trove of potentially sensitive American intelligence data, including the names, photographs and telephone numbers of Afghan spies informing on the Taliban and Al Qaeda. The flash memory drive, which a teenager sold for $40, holds scores of military documents marked "secret," describing intelligence-gathering methods and information — including escape routes into Pakistan and the location of a suspected safe house there, and the payment of $50 bounties for each Taliban or Al Qaeda fighter apprehended based...