ESXi 5.5 prevents HDD spin down

Hello!

First, sorry for my bad english!

I have a problem with my ESXi 5.5. I am running a VM as NAS with Openmediavault.

I am using four HDDs which the host accesses as physical RDMs. (http://www.vm-help.com/esx40i/SATA_RDMs.php)

S.M.A.R.T-monitoring in the VM works fine and i can e.g. spin down the HDDs manually with "hdparm -y /dev/sd[bcde]".

But i want an automatic spin down after 2 hours of idle. But it appears that the ESXi accesses the HDDs every 30 min, because the automatic spin down only works with an idle time less then 30 min.

Do you have any ideas how i can turn off this disk access or increase it at over 120 min?

Thank you very much!