I use the USB Backup option to provide off-line backups of my EX4. Small-ish backups proceed correctly with the status bar updating and, when completed, a “Backup Complete” message on the GUI.
However larger bckups never receive the “Backup Complete” message, instead they show “Cancel” on the GUI. From mounting the backup volume and perusing the contents, it appears that the backup has, in fact, completed but this is never stated.
The way to remove the “Cancel” status message is to “Edit Job”, then “Save”. After that the GUI states “Ready”