While I'm sure not a USB or CHDK expert, someone who develops software for USB send me a link to the tool that he uses: SnoopyPro (
SourceForge.net: Usb Sniffer for Windows). It is able to capture messages between a PC and any USB-device.
For using SnoopyPro.exe:
* Press F2 for devices
* File -> Unpack Drivers
* File -> Install Service
* Right-Click on your camera name and select "Install and Restart"
* If camera software is installed then wait popup window. Some PCs starts to work very slowly (waits several minutes) when SnoopyPro is capturing - be patient.
* Start generating USB traffic (e.g. using remote capture or reading pictures from the camera)
* On SnoopyPro window press Stop
* Analyse or save the USB packages
It might give answers on what USB messaging is done by mass storage devices, and how Canon communicates via USB. This might give some form of information how Canon handles USB inside the camera (with firmware).
Would this be of any help??