Upgrading to ESXi 5.1 - Best Practices
Source: Global Knowledge
If you are upgrading to ESXi 5.1, there are some important facts that you should consider first. Upgrading involves many stages and processes that must be performed in a specific order. Many of these processes are one-way and do not provide a "back button". If you do not use care and consideration in your upgrade plan, you could possibly lose important data and configuration; and potentially even lose contact with your servers.