Can't eject physical disk

I have an external USB disk with two APFS partitions that I use for backup. I can eject these partitions, using the Finder or the Disk Utility, but nothing I try (including eject commands from the command-line/shell) lets me eject the disk that contains these preconditions. I get a display like the above.

and from here I cannot eject the disk that contains these partitions, even though none of them are mounted. I press the eject icon shown above, and it responds by blinking the icon background, showing that the press has been registered, but nothing happens -- the disk doesn't eject, and there's no error message.

There isn't anything obvious in the Console logs, either, but I don't really know what I am looking for in there.