You can attach virtual hard disks to a SCSI controller to increase the amount of storage available to a virtual machine. ...

You can attach virtual hard disks to a SCSI controller to increase the amount of storage available to a virtual machine. Install the integration services in the guest operating system to improve performance when using storage attached to a SCSI controller. Do not attach a system disk to a SCSI controller. A virtual hard disk that contains an operating system must be attached to an IDE controller.