A bit red faced, but it turns out that if I plug one of the drives
directly into one of my MBP's USB ports, everything is fine (including the presence of eject buttons and the availability of the "When you connect" option in SD!).
The problem is only fully replicable if I plug a drive into my D-Link 7-port powered USB hub, and subsequently the hub into my MBP.
I know this set up of launching SD! when one of these same drives was connected via the D-Link
had actually worked before, but I cannot be sure when it stopped working. My guess is the install of El Cap but, again, I'm not sure.
Anyway, thought I better post back with the final solution in case anyone encounters this going forward.
Thanks again, Dave.