Hello & welcome, Stuart !
I am new to all this but have been playing with RAW files just to see how it works. My SX10IS seems intent on saving both CRW and DNG files when set to save RAW format files. Nothing I do seems to stop this from happening. Is this a known bug in the SX10IS CHDK or is there some magic incantation I can use to stop both being created. The problem is that it is creating 30gb of data (2x15gb) for each photo taken?
No, none of the known CHDK versions creates both, CRW and DNG...
a) When RAW is enabled, RAW images are created, with the filename extension you've choosen in CHDK (e.g. CRW or CR2)
b) When DNG support is enabled, the DNG RAW images will be stored with the same filename extension (CRW ord CR2), but the images are in the DNG format.
c) When you enable the DNG filename extension, then (and only then !) the DNG RAW images will be stored as .DNG on the memory card.
I guess, your problem is that it LOOKS LIKE there are 2 files when you view/transfer the images via the USB connection !?
If so, enable DNG support, set the filename extension to .CR2, disable DNG extension; also do a power off & on cycle before you connect the camera to the USB port.
Otherwise use a card reader to transfer the images...
PS: the images have 15MB, not 15GB 
*EDIT: reyalP was faster 
