SCSI Voodoo it was.

The bus had been going:

[internal drive 1 - internal drive 2]{CD-RW 4 - Zip 6 - external drive 3 - MO 5}

...with [] being the internal bus and {} being the external bus. The entire bus worked when reorganized to:

[internal drive 1 - internal drive 2]{Zip 6 - external drive 3 - MO 5 - CD-RW 3}

The only change was a HD50 cable with an HD50-Centronics 50 cable that was longer. Bizarre. :-/