Thanks all,
I have tested the the drives. With hdparm everything works fine. The only Setting that is working for all of my drives (Samsung and WD)
APM: 255-Disabled
AAM: Disabled
Spindown time: 30 minutes
Than the Samsung going into sleep after 30 minutes and the WD after around 25 minutes (a little bit earlier).
Also Plex was preventing one of the Samsungs going into Spindown mode. But that was easy to find out.
Now I want to shrink the SSD (120GB) OMV partition and set the plex Library to the SSD. Hopefully it will work.
Thanks!