Add additional hard disk VM, In this article we are going add a virtual hard disk to an existing virtual machine, or you can add a hard disk when you customize the VM hardware during the vm creation process. When we add the hard disk to a virtual machine, you can create a new virtual disk, add an existing virtual disk, or add a mapped SAN LUN.
Add additional hard disk VM (Virtual Machine).
1- Login to your VMware vSphere Client, In the vSphere Client inventory, right-click the virtual machine the one you want to add an additional hard disk and then select Edit Settings.

2- Click the Virtual Hardware, click Add New Device, and then click Hard Disk under Disks, Drives, and Storage.

3- After selecting hard disk, Now you can see new hard disk has been added.

4- Select a location to store the disk. Save with the virtual machine or specify a datastore. If you selected to identify a datastore, browse for the datastore location.

5- Select a datastore where you want to store and then click OK.

6- Thick Provision creates a virtual disk in a default thick format.
The Thick Provision Eager Zeroed creates a type of thick virtual disk that supports clustering features such as Fault Tolerance.
Thin Provision creates a disk in a thin format. Select this format to save storage space.
Disk provision selects Thin Provision.

7- Click ok.
