Mike, by 2" cement floor, do you mean a mortar bed; it's too thin for a slab on grade. Cement board wasn't a very good choise if this was a renovation. when cement bd. is used, it is supposed to be set in a bed of portland cement mortar. I doesn't sound like your's was (not to supprising). The best thing would be to remove it, but of course you're looking at a whole new floor and the necessity of dealing with a height change. If you were going to do the floor anyway, loose the backer bd. You'll have to make the tradeoff.