MBP unable to use Time Machine without Cinema Display

While I would be surprised if I was the first person this happened to, I can't seem to get the right combination of search terms or something, because I can't find any other discussions about this same problem.

The problem is this:

I have a 2019 MacBookPro (Mojave), a 1TB G-Technology (Western Digital sub) USB 3 external drive, and a Cinema Display. Time Machine is configured to use the external drive. It worked fine when the drive was attached to the Cinema Display. Every attempt to read the drive (Finder, `ls` from iTerm2, `tmutil inheritbackup` from iTerm2) when it's now attached directly (via a dongle, that is) to the MBP leads to a hung Finder/shell. Ditto trying to execute a Time Machine backup when the drive is attached directly.

One fun wrinkle is that if I attach the G-Technology drive to another machine, I have no problem navigating the drive's contents via the finder.

Another is that in exploring using `inheritbackup`, I believe I disassociated the MBP from the backups on the drive. This doesn't seem to spell doom, as long as I can eventually read/write the drive from the MBP and use `tmutil` to reassociate the machine with the backups, but feel free to correct me.

One final point, whether it's meaningful or not: The laptop was away from the drive for 10 weeks or so (thank you, COVID-19), and when I first reconnected the drive to the laptop (not via the Cinema Display) the laptop wouldn't recognize the drive at all. I got the MBP to recognize the drive only by resetting the SMC.

Whew. That feels like a lot of info, and I don't know whether it's all relevant. I definitely don't know where to go next, so any constructive advice is welcome. Assume that I know my way around a shell, but also that I'm no leet hax0rs.