The download to the brick may also fail with a message like this:
C:\Java\lejos\bin>lejosfirmdl -f Use --help for options. segment.length = 13686 (14k) segment.length = 2938 (3k) Total image size=16624 (17k) C:\Java\lejos\bin\lejosfirmdl: no response from rcx
I intentionally created this error on my system by pointing the infrared tower away from the RCX brick. This is easily corrected by pointing the infrared tower at the top of the brick.