The upgrade procedures differ for ESX 4.0 and ESXi 4.0.

Caution

You must upgrade to vCenter Server before you upgrade ESX/ESXi. If you do not upgrade in the correct order, you can lose data and lose access to your servers.

When you upgrade a host from ESX 3.x to ESX 4.x, the customized partitions in 3.x are not replicated in 4.x.