Diskpart may fail when creating more than one partition when executing the “assign letter=D” command. This is because Windows, WinPE included, by default assigns the first CD-ROM “type” drive to the first available letter after C:. So if you are partitioning a new server, or repartitioning a server that contains only 1 partition (C:), the CD-ROM drive will be assigned to D:. Therefore, if you were creating more than one partition and wanted to assign letter D: to the second partition, it would fail.