USB HDD Why won't the spindle go to sleep?

Question

linux

There is an external HDD USB 3.0 namely Seagate 4tb. when connected to an iMac, in the absence of disk access, you can hear how it parks the head and stops rotating, and there is silence.
I assembled here a self-made NAS from Raspberry pi4 installed debian 9 and openmediavault there.
Everything seems to be fine, but I hear with my ears that the spindle has not stopped for 2 days already. I connected via SSH and wrote this:
sudo hdparm -s240 /dev/sda

I get answers
SG_IO: bad/missing sense data
Well, in general, I asked 7 minutes to stop, but it didn’t work, the spindle still spins and does not go to sleep.
What to do? There are no accesses to the disk via the network (I think so) and, according to the idea, nothing can prevent it from going to sleep.