I have an ESXi 5.0 server running here at my house which runs on a box with a single 2TB Hard drive and one datastore. I wanted to see if I could add a second hard drive and make a second datastore. Here are the steps I used to make it happen.
Stop the server install a second hard drive and rebooted the ESXi 5 server.
Open up vSphere Client and connect to the ESXi server.
Select the Server.
Click on the Configuration tab and click on Storage.
Click on Add Storage.
And without doing any searches, my hard drive just showed up. Select the Hard drive and click next.
Give it a name and click Next.