Using a Nodemcu and a relay, I can simulate unexpected removals of a USB drive by interrupting the positive voltage line in a USB extension cable.

The test is defined as: Keychain backup turns on, mounts the USB drive's encrypted container. A script running on another computer ssh's in and verifies its mounted. The script then starts a rsync backup and waits. After a few minutes, the script sends a command to the nodemcu which turns the relay off, abruptly disconnecting the USB drive. After 10 seconds, it turns it back on and the test repeats.