I am a relative newbie, but I guess I am onto something here.
I noticed, that in an old topic "DNG visible vis USB not working in Win7x64?" by larry.charlot.31 « on: 27 / July / 2013, 22:04:29 », the non-functioning of the option 'DNG visible via USB' from the RAW menu had been reported. This message had only 1 reply which came to mentioning that the OS was irrelevant for this to function.
However, I experienced exact the same problem.
The RAW pictures that I made with a A720IS were NOT visible on my Win7 x64 Ultimate machine, but they WERE vsible on another Win x32 XP machine (without changing anything in the camera menu settings within CDHK. The version of CHDK that the camera is running is a720-100c-1.2.0-3350-full
As the machine for which this problem was reported earlier ALSO had the win7 x64 ULTIMATE OS, I suspect it HAS something to do with the OS, but there is more...
I run CHDK also on an A570IS, this is version a570-101a-1.2.0-3155-full, but this camera has no such problem at all. Under Win 7 x64 Ultimate it DOES show the RAW files as it should. Now I can say that although the OS has something to do with the mentioned problem, there is also another isuue that plays a role, I suspect it could be the CHDK version, but more likely it is the camera model that comes into view as the forementioned poster that reported this problem before me ALSO ran CHDK on this model camera: A720IS !! (and that poster ran a much older version: Build 0.9.9-1470).
Could it be that something went wrong in the development of the CHDK versions that gives rise to the malfunctioning of the combination A720IS and Win 7 x64 Ultimate?
I will come back at this issue after trying more OS types and cameras, but whenever someone else has something on this issue I am glad to become aware about it ...