Most hard drives for the 64/128 plug into the CARTRIDGE port, at least here in the States.... and that's a 44 pin connector. The Plus/4 and 16 use a 50 pin cartridge connector.
And although Commodore claimed when the Plus/4 came out that the USER port is the same as the 64, there are a couple of pins that have to be swapped if you want the Plus/4's ACIA to detect things correctly. (offhand I don't remember which pins these are, sorry)