In "Q Who?," the Borg are seen using a laser to cut out a section of several decks of the saucer section which they try to retrieve with a tractor beam.
WORF: A type of laser beam is slicing into the saucer section.
RIKER: Carving us up like a roast.
As a result of an earlier scene where two Borg drones interfaced with a computer in engineering, it's reasonable to assume that the Borg have access to detailed schematics of the Enterprise, so it's reasonable to assume there was something in that section which the Borg wanted.
What was in the section of the ship that they removed?