Quick follow up to my earlier post, any drive that's a a laptop or desktop drive in an enclosure will not work, it needs too much power and the Linx will not be able to supply it. You may see lights come on, however it will not mount under Windows.
External USB drives (i.e. dedicated drives you can buy not ones in enclosures) _do_ work via the OTG cable. I've tested two now and they power up and work correctly with my Linx 7.
USB memory sticks - no problem with any I've tried, they all work.
Hope that helps