Porter's question is a particularly relevant one. The Mac Plus does not supply termination power. Termination resistors perform two functions -- they reduce transmission-line reflections from bending the bits, and they also provide a DC return path to assure proper logic levels. The DC, in turn, has to be supplied by something external to the Plus.

Drives generally have a variety of jumper-controlled options for how termination power is to be supplied (e.g., from the host computer, from a local power supply, etc.). If it is currently set to receive term. power from the host, then the system will behave much as you have described.