Okay, I solved it.
For those in the same situation:
From Linux Disks application, "Drive Settings" menu, enable "Apply Standby settings" for each of the 5 drives.
Now the D5-300C stop all drives and fans when the pc is turned off or suspended.
I just bought a D5-300C and filled it with 5 single hds. When connected to a GIGABYTE BRIX MKLP3CP-00 running Linux Mint 21.2 Cinnamon, the D5-300C never goes to sleep, even an hour after the BRIX has been turned off, all 5 drives are on and the fans are spinning. Connected to a PC with Windows 10 e...