No HDD spin down on Odroïd HC1 with NCP

I have nextcloudpi installed on an Odroïd HC1, with a 2TB 2,5” Seagate drive.

Everything works fine, but I cannot make the HDD spin down when there is no activity on the cloud.

Apparantly, I’m not the first one that experiences problems in that respect. After reading a lot of posts on different forums I still haven’t found source of the problem.

What I already did:

  1. Upgraded the firmware of the JMS578 usb3 to SATA-bridge, according to the instructions of Hardkernel

  2. I tested the HDD with HDParm
    sudo hdparm -y /dev/sda
    returns

/dev/sda:
issuing standby command

sudo hdparm -I /dev/sda | grep ‘Write cache’
returns
* Write cache

So the drive can do write cache

sudo hdparm -B127 /dev/sda
does make the drive spin down but only for about 20 seconds. After that it’s spinning up again.

This must mean that something is keeping the drive active, but I cannot figure out what. Could it be something in Nextcloudpi or Nextcloud itself?

Any help would be very welcome.