In my opinion the main issue here is power. Camera does not have enough power to drive external HDDs. That means, only HDDs with external power supplies could be connected. This limits the range of external drives which can be used + severely limits usefulness of such solution - you must have power outlet to plug your HDD power supply. Of course there are specialized HDD enclosures with batteries, however such HDD usually have card readers and there's no need to connect them to camera
Also in USB connection one device must be master another slave. All HDDs are designed as slaves, so, cameras must be able to work in dual mode: master when HDD connected, slave when camera connected to PC. Of corse this is doable.