To begin, this Apple Discussions thread talks about "double termination," which is what I was warning against:

http://discussions.apple.com/message.jspa?messageID=2748302

Obviously, if you put more than one device on the external SCSI connector of the PLUS, you will have to terminate the first and last device. But my previous post here was written under the assumption that the HD20SC would be the only external SCSI device attached, hence it needs only 1 instance of termination (either via jumper on the drive itself, or a terminator connector block externally attached to the HD20SC). This page shows a photo of the "Centronics" terminator connector that I've been talking about, which attaches externally to the HD20SC (and many other external SCSI hard drives):

http://www.jagshouse.com/plusharddrive.html

As to "termination power," you're right about the Plus NOT supplying it. However, most Apple SCSI drives provide it so they can work on the Plus:

http://lowendmac.com/tech/termination.shtml

With that said, I've not verified this myself because I don't have a Plus.

In the past, I always preferred to use Active Terminators because they supplied termination only when needed:

http://www.welovemacs.com/scs-term.html

It therefore would be good to test the following 3 conditions (all with the HD20SC being the only item in the external SCSI chain):

1) Jumper based termination on the drive mechanism itself (if the drive offers it).

2) A "passive" Centronics terminator externally placed on the HD20SC.

3) An "active" Centronics terminator externally placed ont he HD20SC.