Add a Hard Disk to a Virtual Machine..............

When you add a hard disk to a virtual machine, you can create a new virtual disk, add an existing virtual disk, or add a mapped SAN LUN.

Procedure

1.  In the vSphere Client inventory, right-click the virtual machine and select Edit Settings
2.  Click the Hardware tab and click Add..
3.  Select Hard Disk, and click Next.
4.  Select the type of storage for the virtual machine’s disk, and click Next

You can store virtual machine data in a new virtual disk, an existing virtual disk, or a Mapped SAN LUN. A virtual disk, which appears as a single hard disk to the guest operating system, is composed of one or more files on the host file system. Virtual disks can easily be copied or moved on the same host or between hosts.
Options
Create a new virtual disk

•   Enter the disk capacity.
•  Select a thin disk format or a thick disk format to allow the disks to support clustering features.
•  Select a location to store the disk. Store with the virtual machine or Specify a datastore.
•  If you selected Specify a datastore, browse for the datastore location, and click Next.

Use an Existing Virtual Disk

•  Browse for the disk file path and click Next.

Raw Device Mappings Gives your virtual machine direct access to SAN.

•  Select the LUN that you want to use for the raw disk, and click Next.
•  Select to store the LUN mapping files on the same datastore as the virtual machine files, or select a different a datastore, and click Next.

5. Select the compatibility mode.

• Physical allows the guest operating system to access the hardware directly.
• Virtual allows the virtual machine to use VMware snapshots and other advanced functions.

6. Click Next.
7. Accept the default or select a different virtual device node.

8. (Optional) Select the virtual disk Independent mode and select an option.

Persistent: The disk operates normally except that changes to the disk are permanent even if the virtual machine is reverted to a snapshot.

Nonpersistent: The disk appears to operate normally, but whenever the virtual machine is powered off or reverted to a snapshot, the contents of the disk return to their original state. All later changes are discarded.
Independent disks are not affected by snapshots.

9. Click Next.

10. Review the information, and click Finish.

That’s it... :)