A method for migrating a virtual machine disk (VM disk) from first physical storage to second physical storage while the virtual machine (VM) is running, the method comprising: (a) taking a snapshot of the VM disk as represented by a first parent VM disk stored on the first physical storage, whereby a first child VM disk is created on one of the first or second physical storage; (b) copying the first parent VM disk to the second physical storage as a second parent VM disk; (c) re-parenting the first VM child disk to the second parent VM disk; and (d) consolidating the first child VM disk and the second parent VM disk.