I have an ESXi 5.0 server running at my house, see http://www.whiteboardcoder.com/p/esxi-server.html, which runs on a box with two hard drives, a 1.5 TiB and a 2.0 TiB. I really need to build an entire new ESXi box, but I also have a wife, two kids, and right now a new ESXi box is way down on the list. (Besides I need to wait at least 6 more months for ESXi 6.0 to be available and proven in the wild)
In the meantime, I bought a new hard drive a 4.0 TiB Western Digital WD40EZRX, which should help me limp along until then.
Turn Everything Off
Make sure to back up anything that needs backing up.
Go into each machine and power it off (A little anal, but safer)
Then open up VSphere and Turn off all your virtual machines. Select the machine and power it off.
Right click on the server and select Shut Down.
Power down your physical server and install the additional hard drive.
Bring it all back up
Bring the server back up and make sure all your VMs are working as intended.
Create a Datastore
Now it's time to turn the new Hard Drive into a new Datastore.
Open up VSphere and select the ESXi server.
Open the Configuration Tab.
Click on Storage then click on "Add Storage"
There is the new hard drive.
Select the hard drive and click next.
Give it a name, I called mine datastore3, click next
It's Ready to go start the set up. Click Finish
In the Recent Tasks section of the VSphere interface you should see the datastore being created.
It did not take very long < 10 seconds.
And there is my third datastore J