External USB drive with zfs pool won't go to sleep (most of the time)

I have an external hard disk with a ZFS pool on it. The data is accessed very infrequently so I would like to drive to go to sleep after some inactivity, which I set to 10min: hdparm -S 120 /dev/disk/by-id/usb-Seagate_Expansion_Desk_NAABDT6W-0\:0 With a cron script I check the status of the drive over time: hdparm -C /dev/disk/by-id/usb-Seagate_Expansion_Desk_NAABDT6W-0\:0 This works sometimes but most of the time, the drive remains in "active/idle" all the time, even if no access occurs. The ZFS pool is not performing scrubs. There are no zvols. All datasets (some of them encrypted) are mounted to under /zpseagate8tb/ (and /zpseagate8tb_bind/ for use in LXC containers). I confirmed no file descriptors to any of the datasets are open: # lsof -w | grep zpseagate8tb # What else could be preventing the drive from going to sleep?