I am helping my colleague debug his problem. Basically, there are two 10M devices, and only one of which we have control over (the other being a "black box"). Analyzing the Ethernet hardware signals with a scope shows that, when the two devices are directly connected to one-another (without going through a switch), the "black box" device sends packets that do not have the 7-byte Ethernet preambles.
What sort of traffic between two Ethernet cards could have no preamble? Is it possible for two network cards to negotiate removing the 7-byte Ethernet preamble? Below is a capture of the network signals we are seeing when the two devices are directly connected: