Okay, I've rewired my dock to use the AC plug for power, and it still fails. The Mk2 works perfectly, the Mk2a loses ethernet just after successful DHCP negotations conclude.

So, I rearranged "the other end" -- plugging the dock into my other Hub (slightly newer LinkSys than the first one), and .. it works now.

Weird. Must be a slight difference in the ethernet implementation between Mk2 and Mk2a..

-ml