Cannot access USB HD through a hard wired PC

I have no problem access my external hard drive that is connected to the router via USB but any PCs that are wired into my network cannot see or access the external Hard drive.

I have connected the router to a switch that other PCs access.  They have no problem getting to the printer nor internet or even seeing the HD of the wireless deivces.

I just need to know what I am doing wrong with trying to access the HD connected to the router.

The PCs are either XP, Vista or Win 7

Thanks