Identified a tiny risk - with the original procedure, you will be removing the card while it's powered on (edited the original post). Canon bootloader unmounts the card *after* the decryption, right before executing the firmware updating code; that is, after those ~ 10 seconds when you can remove the card. While it's probably OK, I can imagine some cards may not like a sudden loss of power, so there is some slight possibility of hardware damage (correct me if I'm wrong).
Another quirk: when using a CF card to perform the update, the card will be accessed
without LED activity (remember the 5D2/50D?), so you won't know when to remove the card. So, if you follow the original procedure, I highly recommend an SD card.
In any case, if you remove the card too early, you will do so in the middle of a data transfer. The bootloader will be reading from the card (nothing will be written during that access), so I don't expect filesystem corruption. After an interrupted read, the decryption process will fail, so there's no chance to end up flashing an incomplete firmware, even by mistake. I wouldn't exclude a tiny chance of hardware damage, but again, I might be overreacting here.
It's probably a lot more likely to physically damage the SD card while trying to perform the swap in a hurry.
Considering the above, I strongly prefer the second method (also summarized
here), which doesn't have any of these risks. Here's a longer description:
- copy firmware 1.3.6 (5D300136.FIR) to the card
- launch Update Firmware from Canon menu, click OK
- open the battery door ASAP,
but don't remove the battery! - if you did it right,
the camera will turn off (wait for a few seconds to make sure it's really off)
- if you see the Firmware Update Program Loading screen, it means you have opened the battery door a bit too late;
wait until it disappears and try again!- open the card door and remove the card from the camera (
do not close the battery door; also leave the power switch on)
- copy firmware 1.1.3 (5D300113.FIR) or 1.2.3 (5D300123.FIR) to the card
- you may leave the original 1.3.6 FIR on the card, or you may delete it; doesn't matter
- put the card back into the camera, close the card door
- close the battery door; you should see the Firmware Update Program Loading screen
- confirm the firmware downgrade from 1.3.6 to 1.1.3 / 1.2.3
-
whatever you do, do not remove the battery in the middle of a firmware update!
That's it. The only tight timing is when opening the battery door; afterwards, the camera will be off, so you can take your time, no need to rush swapping the cards.
To answer a question from another thread:
Can this possibly be used on all Canon cameras?
Yes, I expect this (second method) to work on all current EOS cameras, from DIGIC 2 to DIGIC 8. The only assumption is that, after clicking Firmware Update and opening the battery door right away, the camera will turn off, rather than restarting (and having the bootloader execute the firmware update file). Following
this investigation, this is likely to happen on all EOS models.
The original procedure (swapping the cards while the update is loading) depends on what exactly the firmware updating code is doing - will it always re-read the firmware file from scratch? I don't know. It will probably work as well.
