This issue has been discussed in a number of places, and it seems that the problem is indeed caused by the extra power from the battery flowing through to the micro-USB connection. My solution was to isolate this power by modifying a USB host cable (as suggested by others elsewhere). I was a little unsure at first, but it turned out to be quite easy to do, so I thought I'd share.
Firstly, I bought a cable on Ebay for about $8. This one has transparent ends which make it easy to see what you're doing, but I don't think it matters which one you get.
The next thing I did was to use a sharp penknife to cut horizontally through the plastic along both sides of the female end, for about 2.5cm (1 inch). This allowed me to bend the cover back to expose the wires.
The power is carried by the red wire, so I cut that to break the connection.
Then I just put a little glue along each of the sides that I'd cut, and you can barely tell that the cable has been modified. It all took me about 10 minutes to do, and works perfectly! Now the Archos 70 IT detects my HDD as an external device every time.
One thing to note is that since there is no power flowing through this cable you cannot use it for any devices that don't have their own power.
