Just a quick follow up, in case anyone else finds themselves in this situation. I wasn't seeing any bad drives because one of the drive doors had accidentally been popped open while moving a heavy cable. Nothing in the Physical Drive list showed a problem in the GUI app. But I took a look at the subsystem report and happened to notice that PhyDrvOnline was 7. 7? Obviously it should be 8. And sure enough, looking in the GUI, there were 7 nice green status indicators, and nothing at all for the 8th. I then took a close look at the enclosure and discovered the 8th drive's door sitting very slightly ajar. I closed it, and *now* the 8th drive showed up everywhere (both the GUI app and the command-line promiseutil), but it showed up as "dead". Since I was almost entirley certain there was nothing wrong with the drive, I forced it back online (using commands below), rebooted the unit (after ejecting from the Mac, I pulled the Thunderbolt cable out, waited for the drive to shut off, then reconnected the Thunderbolt cable), and all is well.
Forcing a dead drive back online is not something you should do lightly, and only on the condition that you are genuinely confident the drive is okay. To do it, you run promiseutil from the command line, then enter:
phydrv -a online -p <drive#>
As I have multiple Pegasus# enclosures and it was the second enclosure that had the problem I had to first enter:
spath -a chgpath -t hba -p 2
And before I forced the drive online I made certain I was dealing with the right unit and had the right drive # by typing:
phydrv -a list
At first the just reconnected drive showed "Dead" for its status. After being forced online it showed "Forced On". After rebooting the enclosure, it just says "OK". And I think it really is.
I don't know if it was necessary, but I then went into Background Activities and kicked off a Redundancy Check on the unit, just to be safe. I also bumped up its priority to "High".
P.S. Promise folks... You really should make the GUI show the slot and mark it as empty, in red, when the drive is entirely missing or the door is open like this.