I am using docker toolbox on Windows 7 to run docker. (docker version 1.9.1)
As it turns out, the docker machine creates its VM at C:\\Users\\username\\.docker\\mach
You can move .docker directory to another drive and create a junction point to it.
Please note that regular shortcut will not work.
For example:
Move .docker directory from C:\Users\username to D:\
and run:
C:\Users\username>mklink /j .docker D:\.docker
Junction created for .docker <<===>> D:\.docker