... the Mac Plus never boots, standing with an exclamation point ...
Your post reads as if you are enjoying termination problems. Older Quantum drives used three in-line resistor packs on the drive's logic board for 'passive' termination. SCSI ID was also fixed at 0, because the drive was conceived as being the only, and therefore also the physically last, drive in a daisy-chain that was inaccessible inside a never-to-be-opened box.
Time and taste moved on. Apple conceded that more drives in a daisy-chain might be permissible, and Quantum conveniently produced drives on which there were header blocks with three pin pairs, A0, A1 and A2, with which to select SCSI IDs from 0 to 6. SCSI 7 is reserved for a host controller, which is the Mac's controller here. ICs were added for 'active' termination, which was enabled or disabled with a jumper on an additional pin pair labelled TE. If such comparatively sophisticated drives and the older drives were placed in the same daisy-chain:
1) the older drive's terminating resistor packs had to be removed if it was not the physically last drive in the chain, but its SCSI ID could not be changed from 0 to anything else, or
2) the newer drive's terminating ICs had to be disabled if it was not the physically last drive in the chain, but its SCSI ID could be set as any one from 1 to 6.
It may be that your Quantum Fireball ST2.1S was/is not terminated, either with on-board resistor packs or with an in-place jumper. Hence the point interrogation when it alone is connected to the Plus
and there is no external plug-in CN-50 terminator on the box. Conversely, the Quantum Fireball SE4.3S has a TE jumper, and works regardless of the presence of a CN-50 terminator on the box.
If you daisy-chain the two drives, fortuitously with the Quantum Fireball SE4.3S at the end of the chain, the lack of termination on the ST2.1S is just what you need when that drive is not physically the last in the daisy-chain, and it is therefore 'seen' by the system. This also argues that the SE4.3S drive is set with jumper(s) for a SCSI ID other than 0 or 7, which is also a fortunate happenstance.
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