I have the same setup, one hdd, 2 xbox's. I was told this shouldn't be a problem as far as getting flagged or banned goes. I don't believe the dashboard is saved to the hdd. My online (unmodded) xbox has the newest update, my offline (modded) xbox has the last dashboard on it and I switch my hdd between the two. The only thing I have noticed is my avatars don't show up anymore on the modded xbox with the older dashboard. Other than that I haven't noticed any problems.