I did the following and it seems to work for me.
- Go to the current Docker image location
- First you need to make a directory on your external storage device.
- Rsync the current dir to the new location and remove it.
sudo rsync -a com.docker.docker/ /Volumes/BackupDrive/com.docker.docker
rm -rf com.docker.docker
- Create a symlink to point to the new location
ln -s /Volumes/BackupDrive/com.docker.docker/ com.docker.docker
- Start Docker for Mac