1. A method for performing disk migration from a first virtual computer system comprising a first virtual hard drive to a second virtual computer system comprising a second virtual hard drive, said method comprising: establishing a file in a file system on an external storage device that corresponds to both the first virtual hard drive and the second virtual hard drive where, initially, only the first virtual hard drive is mounted and accesses said file; and dismounting the first virtual hard drive in the first virtual computer system and subsequently mounting the second virtual hard drive in the second virtual computer system whereby only the second virtual hard drive accesses said file.