After you add a volume of external storage to a VM, save the VM before you modify the volume.
To modify a VM volume:
Select from the list of controllers: IDE, SCSI, SATA (ESXi), VIRTIO (KVM)
ESXi only. The user can enter one of the controllers supported by their platform, which may include:
To make other changes to a volume (e.g. resize), go to the Virtual datacenters view on the Volumes tab. See Manage Persistent Storage and VMs#Managepersistentvolumes