Are they plugged into a USB Hub? If not Try plugging them into different USB busses (ie one in the back of the PC and one in the front) Sometimes HDDs do use power from the USB even when they're plugged in and this can cause them not to recognise a usb hard drive if they're both in the same bus. Also do either of the HDDs work seperately? When you plug them in put it up to your ear, if it makes a regular clicking/ticking sound, it means the drive is damaged and if it makes a whirring sound and a couple of irregular clicks it is powering up ok.