Thanks. I think you may have misunderstood my question, which is a generic question about SCSI and not really about the Plus. Basically, there must always be two points of termination at opposite ends of a SCSI chain, right? So in a Mac that only has an internal HD, where are those two points?

Now add an external SCSI device to that same Mac. Since it's external, it forms the new end of the chain, and must be terminated. Now you will have three points of termination unless you either manually change something inside your Mac, or the Mac auto-adjusts somehow. Since in reality no manual internal changes are needed, that implies that the Mac auto-adjusts somehow, but I don't know how.