I gave up trying to figure out why, but it appears to be related to the lack of hardware handshaking in the SCSI implementation uniquely used by the Plus.

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Interesting ... here's a question: the Plus was the first implementation of SCSI before the standards were set. The last upgrades to the Plus logic board, as far as I know were with the 2nd gen. ROM in 1986. Yet the Plus was manufactured until almost 1991. In the meantime, the SE came out. Now I know the Plus was Apple's low-end entry-level, cash-cow Mac, but wouldn't it have benefitted them to bring the Plus' SCSI implementation in line with that used on the SE and every other Mac? Was it so different that a few minor tweaks on the Plus board couldn't have done it economically? Would a ROM update alone have done it?