The easiest way to change the default save location of the Camera app:
Snap a picture with the Camera app to create the default Camera Roll folder.
From File Explorer\Libraries\Pictures move the Camera Roll folder to the desired location, including removable media - simply drag and drop.
Done!
For the curious the registry key is:
HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders. *edit - no space in "Explorer"
Value name: {AB5FB87B-7CE2-4F83-915D-550846C9537B}
Value data: path for the desired location e.g. D:\Pictures\Camera Roll
This key does not exist by default, you'll have to create a new Expandable String Value (the key is created if you move the Camera Roll via File Explorer).
Hope this helps