Hdparm and hd-idle don't spin down a USB3.0 enclosed HDD


I have a USB3 enclosure attached to my router.
I noticed it never spins down. I don't think it's a problem of a network client using it, and not letting it idle.
I have disabled all the network shares, and disabled the minidlna service. unmounted the partitions, and 30 minutes later de drive is still spinning.

hdparm doesn't seem to able to set the spin down time.
hdparm -S 120 /dev/sda1
Returns: SG_IO: bad/missing sense data

hd-idle doesn't seem to work either, I tried it for days, and the drive never spins down.
If I ask the drive to spin down immediately with
"hd-idle -t sda" it does not spin down.

Are there any solutions I could try? I am not sure I understand how hd-idle is supposed to work, and spin down the drive.
Not sure if anything I can try. Any ideas?