halo jo:

omg i just remembered something. the flood have a means of transportation. they even have a slipspace generater. they have a unsc ship. they have!!!! in amber clad!!!! if they fix that thing up they could load up the pelicans back in the ship, get all the long swords ready to launch and they could attack earth. even if it were damadged they could quickly repair it and load it with flood and boom a full battle ready flood attack carier ready to take over earth!!!!

They could attack lots of places - they most likely know the location of the covenant homeworlds as they control high charity and they may have collected some information from the Truth And Reconcilliation.

Also, with cortana in the clutches of gravemind he could somehow use her to find out the location of  Earth.