For Windows:
%USERPROFILE%/VirtualBoxVMs/boot2docker-vm
You can manually set the storage driver with the -s or –storage-driver= option to the Docker daemon.
/var/lib/docker/{driver-name} will contain the driver specific storage for contents of the images.
/var/lib/docker/graph/<id> now only contains metadata about the image, in the json and layersize files.