Sometimes it’s necessary to only migrate a single hard disk from a VM. This is the case when someone adds two 1 TB VMDK’s and fills them up completely. The maximum size of a VMFS datastore is 2TB minus 512 bytes. So in this case the datastore will be completely filled with no space left to keep the VM running. So if you want to migrate just one hard disk to make sure the VMFS datastore will not fill up. You can use the vSphere client or PowerCLI to do perform this “advanced” Storage vMotion.
Note: if you want to reclaim your “wasted” storage back from your SAN, you have to recycle the whole datastore. So you have to migrate the other hard disks and configuration files as well.
Start the Migrate Virtual Machine wizard and select datastore: