Did the computer work okay before you put the second HDD in?
OK, so your old HDD is installed on the second plug on the same ribbon cable as the main HDD in the new machine, and its jumper is set to slave?
But what about the main HDD? Is its jumper set to master? Some have different jumper settings when they're working as master without a slave than when they're working as master with a slave. Did its plug become loose when you installed the second HDD?