I suggest that you use the alternative firmware updater for OS 2.1:
http://raysfiles.com/os/osFWUpdater_0616.zip
The latest firmware that can still be uploaded through USB to OS 2.1 is firmware 2.1.5. Firmware 2.1.7 cannot be uploaded through the USB port anymore, and the reason is explained here:
https://openthings.freshdesk.com/solution/articles/5000668400-updating-opensprinkler-2-1-to-firmware-2-1-6-and-beyond-
This applies to OS 2.1 only.