Recently we have been running the vCheck powershell script daily in order to help in monitoring our vSphere ESX 4.0 environment. One of the oddities that we have been seeing is that some of the datastores on the SAN don't always show up on every host. Our hosts are connected redundantly, via FC, to some brocade FC switches, which then connect via fiber to our EMC Ax4 SAN. While all the datastores are presented to each host we have, and they see them initially, they sometimes seem to fall off and are no longer visible. It easy enough to rescan for datastores and add them back to the hosts the hosts but this seems to be an error. Has anyone else seen this or know why it may be happening?

Responses to questions:
1. Is it always the same ESX servers that lose their connection? – Scott Warren

No this happens randomly on random hosts. If a VM is running on a particular host, of which the VM's disks are on a SAN datastore, then that datastore won't disappear. It seems to happen if a host doesn't touch a datastore for a bit and it just forgets about it.