Classroom Resource: Recovery from Failed Firmware Update
When a brick fails to complete a firmware update successfully, upon rebooting it get stuck at the "Starting......" screen and never finishs booting. The problem is, the EV3 software will not recognize that the brick is connected in this mode, so there is no way of proceeding with a reinstallation.
The cure for this (thanks LEGO Ed tech support) is to remove and then reinstall the battery to power the brick down. This time, rather than pressing the Center button to power up the brick, press and hold both the Center AND Right buttons to turn the brick on. The brick will now power up in "Updating....." mode and will be immediately recognized by the software when you plug it in. You can then proceed with reinstalling the firmware again.
This technique worked for both my problem bricks, although LEGO Ed says occasionally this method fails and further debugging is required (press the capacitor discharge button under battery #2, try a different computer, reboot computer with brick connected, etc.).