USB 3.0 hard drives don't spin down when "safely removed" from Nautilus

I have a few external drives, some using external power, other being USB powered.

I plug my USB 3.0 drive in a USB 3.0 port.
When I "eject" the drive (from Nautilus) it keeps on spinning.
When I "remove safely" the drive, its name disappears from Nautilus, but the drive keeps spinning. I don't see the point.
I need to physically unplug the USB cable to stop it.

How can I make a drive stop spinning when ejected?

Now, something weird is happening, that might help :
If I plug this same USB 3.0 drive in a USB 2.0 port, it will properly shutdown and stop spinning when I "remove safely" the drive.

Then when I plug it back to the usb 3.0 port, and try to "remove safely", it will... remount automatically! This is really weird, I don't get it.

Last thing I noticed is that the command udisks --detach /dev/sdb does the trick. Why would "remove safely" not do it?

AFAIK, "Remove safely" just unmounts the device, and removes it from udev. It doesn't send any command to the drive to shut down.

On the contrary, udisks --detatch does so: (udisks manpage)

--detach device_file [--detach-options options]
       Detaches (e.g. powering down the physical port the device is
       connected to) the device represented by device_file using a
       comma-separated list of options.