“MigrateDatasourceDataFromDPM script can have many formats one of which’s to migrate a source disk to a target disk or array of disks. How much space do you need to migrate? You need at least as much as the source disk size of course but what if 1 or more volumes span to another disk? Take the sample below; you’ve Disk-1 and Disk-2 (both 750GB) with 4 volumes, note that volume V3 (green) spans Disk-1 and Disk-2. If you want to migrate Disk-1 you’ve to migrate V1, V2 and all of V3 which totals to more than size of Disk-1. Given possible spanning scenarios this’s more challenging than you might expect. If all involved volumes are replica volumes it is fairly straight forward (“Get-DPMdisk” gives you all replicas) but if 1 or more are recovery point volumes it becomes a different ballgame,” writes Microsoft.
The script figures that out and the synopsis explains how it does that. Note the script isn’t tested with colocated datasources.
More Info: TechNet