As stated before, with the boot drive completely wiped and a clean MacOS installed, the flight simulator software still can not receive signals from the USB dongle connecting it to the transmitter ***when booted from this disk***. Reboot into the scratch disk containing another clean installation of MacOS and another copy of the flight simulator software, and everything still works fine, so long as the dongle is in a built-in USB port. No response when it is plugged into the Sonnet PCIe USB card.

New test: ***While booted from the scratch disk***, open the copy of the flight simulator on the boot drive. The sim opens, but, once again, the USB dongle can not connect to the transmitter. Something is preventing the software on this drive from communicating properly with the USB bus. What are the possibilities? bad software, despite the clean installation? bad SATA port housing the drive? gremlins?

One possibility would be to swap what I'm calling the "boot" and "scratch" drives, but the boot drive is faster and newer, and the mystery would remain.