ESXi4.0 stores two build. one is standby and other is boot. When you update host,standby build is updated and boot build is push to standby state. If host doesn’t boot after it is updated, it can be rollback to standby build i.e. which was actually build boot. If you manually boot into standby host instead of current build, irreversible rollback occurs. This means you have to again update the host.Remember esxi permits only one level of rollback.Only one previous build can be saved at a time.