Since the HD7 unfortunately does not provide 5 V power supply at the Micro-USB, you need a Y-cable (eg Delock Adapter Power USB cable 2x USB Y-cable), and an external power supply recourse to connect drives, HDDs, keyboard or mouse ,
But you can also use a powered USB hub.
Presumably, the HD7 has to be rooted to make it work!
In addition, some apps are useful, such as the USB OTG Helper.
It took me some time and energy to find out that the USB devices are only reliably detected when pulling the plug right on the Micro-USB port on the HD7 and reconnected.
Only then the HD7 for new hardware seems to look.
A plugging the unit behind the adapter does not recognize the HD7.
Those who would also like to get involved in the adventure USB OTG on HD7, find all information about this in the thread "USB OTG functions" in the Android forum.