Mounted USB will not spin down

Further investigation:

For testing I’ve changed the spindown idle time to 5 seconds.

hdparm -S 1 /dev/sda1


setting standby to 1 (5 seconds)

And yes, the drive spins down. But unfortunately it starts to spin up right after the spindown. And 5 seconds later it does the same thing all over again.

Setting the spindown idle time back to 15 minutes:

hdparm -S 180 /dev/sda1

setting standby to 180 (15 minutes)

And the drive keeps spinning.

lsof says that there are no processes running on the mountpoint (/mnt/data) or the disk (/dev/sda1).

I also followed these steps to ensure that nothing is running, that could access the drive.

I’m getting more and more the idea that there is some kernel process interfering.

Is anybody else experiencing the same issue?