Background migration of virtual storage

Described is a technology by which a virtual hard disk is migrated from a source storage location to a target storage location without needing any shared physical storage, in which a machine may continue to use the virtual hard disk during migration. This facilitates use the virtual hard disk in conjunction with live-migrating a virtual machine. Virtual hard disk migration may occur fully before or after the virtual machine is migrated to the target host, or partially before and partially after virtual machine migration. Background copying, sending of write-through data, and/or servicing read requests may be used in the migration. Also described is throttling data writes and/or data communication to manage the migration of the virtual hard disk.