Thanks for the answers!
I think I’ve solved my own problem. The cause of my problem that I was using hdparm with partition, not the disk itself. I also totally miss the following information on the documentation:
-B Set the Advanced Power Management feature.
Possible values are between 1 and 255, low values mean
more aggressive power management and higher values
mean better performance. Values from 1 to 127 permit spin-
down, whereas values from 128 to 254 do not. A value of
255 completely disables the feature.
The previous command was like this:
hdparm -B 150 -S 36 /dev/sdb1
I changed it with this:
hdparm -B 120 -S 36 /dev/sdb
And now, it is working as expected. I also found a command that shows the current state of the drivers:
$ ps aux | grep hd-idle | grep -v grep | cut -c 66- ; for f in [a-d] ; do hdparm -C /dev/sd$f | grep -v "^$" ; done
/dev/sda:
drive state is: standby
/dev/sdb:
drive state is: standby