Ouch. My impression is a lot of people have used this model without encountering anything like this, so it's quite puzzling. There were some reports of problems with this camera after a crash with the lens extended.
Once I bricked one camera, I decided to purchase new sd cards on the off chance there was something wrong with one of them.
Can you describe the specific sequence that lead the first camera being bricked? You prepared the card, booted the camera, and it was dead?
When it bricked, were you trying to use the bootable SD card method (card locked) or firmware update?
One camera works when I hit the play button.
Running CHDK with one of the prepared cards, or just with the canon firmware?
The first time I tried them, they would flash the LED above the '?' button, but the screen never would come on.
Did it blink continuously until you pulled the battery, or just briefly black?
I've tried pulling the batteries, I've tried with and without sd cards. The cameras are dead.
Just if case: If you have any USB stuff connected to the cameras, try with it disconnected.
Are you using batteries, or external power adapters?
If you have removed the battery and SD card, and then reinserted the battery, and the camera doesn't boot, that implies something is very wrong with the onboard firmware or the hardware. It
is possible for CHDK to crash in a way that makes the camera look dead, but pulling the battery should fix this. If the camera was left in this state for a long time, the battery could be drained. If there's any doubt, try using the battery from the working camera.
I've attached test build for ixus160_elph160 that should blink the LED as early as possible in the boot process. It should blink continuously until you pull the battery. If you have a prepared card, you can just replace the diskboot.bin. If it blinks, then we know the camera is alive and additional debugging may be possible.
If not, you might have to resort to the
UART to tell if it's alive.
Other questions:
Did you successfully use the cameras before installing CHDK?
Were they new or used?
Do you know if they were original sold for the US market, or somewhere else?
edit:
If you know the specific version you ended up installing, that would also be useful