I've found that each time when I unmount a USB drive, I get a pop-up that says:

Writing data to device
There is data that needs to be written to the device "<device>" before it can be removed.  Please do not remove the media or disconnect the drive

This window does not go away on its own.  It doesn't seem to matter whether I click "Cancel" or "OK", the USB drive appears to remain mounted.

Something is apparently done with the USB thumbdrive because trying to click on the device will yield a new pop-up with the following:

Failed to mount "<device>".
DBus error org.gtk.Private.RemoteVolumeMonitor.Failed:
An operation is already pending.