Digging a little bit, found out ANdré is right and I was wrong. I was also thinking by the AAM x FDM difference, but this is related to the vCenter version NOT to the ESX versions. vCenter until 4.1 installs aam on ESX, and vCenter 5 installs FDM.
As per availability doc: http://pubs.vmware.com/vsphere-50/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-50-availability-guide.pdf page 24:
NOTE ESX/ESXi 3.5 hosts are supported by vSphere HA but must include a patch to address an issue
involving file locks. For ESX 3.5 hosts, you must apply the patch ESX350-201012401-SG, while for ESXi 3.5
you must apply the patch ESXe350-201012401-I-BG. Prerequisite patches need to be applied before
applying these patches.
These patches are required so the ESX 3.5 is able to run the FDM sent by vCenter.
Sorry enkidoe for the wrong information. Thanks Andre for the light.
