Just had same problem "Failed to begin update on the target device. The target device is not in the proper mode to accept an update" when trying to install Firmware v20.013.81 This is a new start up, tried VIA usb and Ethernet both and got same error. Problem was I was in old school mode drilling down through the ip address to the back plane and then selecting the processor. What I should have done was single click select the IP address and start the update (don't drill down). Also important to note I assigned an IP address via bootp and did not leave the bootp server running so when processor rebooted the update timed out. :doh:Tried again leaving the bootp server running and all was good.
Good luck!