When you use vSphere Host Update Utility to perform a host upgrade, the support for rolling back upgrades differs for ESX and ESXi.

For ESX, the default behavior for vSphere Host Update Utility is to roll back the host to the previous version of ESX if the upgrade does not complete successfully. You can override the default behavior by deselecting the Attempt to reboot host and roll back upgrade in case of failure check box in the upgrade wizard. Disabling the roll back allows you to debug your host if the installer fails.

For ESXi, vSphere Host Update Utility does not support roll back. However, after an upgrade, the ESXi host might reboot into the previous version of the firmware if the upgrade version does not boot successfully. You cannot disable this behavior. If the upgrade fails for VMware Tools or the vSphere Client, roll back is not supported and the packages are removed from the host. The packages are removed because insufficient space is available on ESXi for two versions of VMware Tools and two versions of the vSphere Client. vSphere Host Update Utility must delete the previous versions before writing the new versions to disk.

After the roll back, upgrade to ESXi 3.5 Update 4 to obtain the latest version of ESXi 3.5 firmware, VMware Tools and the VI Client.