Plex prevent synology to put the disc in sleep mode.

Hello,
DS 412+
PLex V 1.4.2.3400
DSM 6.1

Plex worked for years without any issues.
Since about two month, even with DSM 6.0 and previous Plex versions, synology do not put discs in sleep mode any more.
I tried to stop Plex -> the discs went in sleep mode as expected. However when Plex is running -> no way.

Is there someone around having same issue or even a sloution to fix that ?