Can Big Sur disable external drives?

Solution 1:

The answer is probably No.  I looked at the USB chain with System Info and found that both machines can see all the USB ports and hubs including the one that I forgot is built into the Seagate drive (which has two unused USB-A ports on it).

I then connected three of them to the adapter hub, and they mounted.

Removed the seven-port hub completely. Plugged the large Seagate and the small older drive into the two-port adapter hub. Then the two other drives into the hub built into the Seagate. All mounted.

So, somehow, the seven-port hub (after more than a year with no problems) prevented any of the drives connected to it from being recognized as drives. 

Puzzling how a hub can provide power and allow something downstream to be recognized as another hub, yet not allow it to be recognized as a disk.  Perhaps that merits a separate question.