DS213J + PSM 0.9.12.18 arm7 - NAS never get back into sleeping mode

Hi team,
I have just installed on my Synology DS213J, Plex Media Server (PSM) latest release 0.9.12.18 arm7.
All is working fine BUT... since this installation, my DS213J never get back into sleeping mode when there is no activity on my NAS nor on any device on my home network. The hard drive activity on the NAS Synology DS213J is always ON as if there were trafic somewhere.
I am afraid it could damage seriously my 2 hard drives.
Do you have any explanation for this behavior ?
Do you have any fix available ?
Thanks in advance for any help.