Comment 10 for bug 1239087

The reason that

 $ udisks --detach /dev/sdX

spins down the disk properly but clicking the "Power off" menu item in the GNOME Disks application doesn't has to do with the fact that the udisks program is from udisks version 1 and was rewritten in udisks version 2.

udisks v1: http://cgit.freedesktop.org/udisks/tree/src/helpers/job-drive-detach.c?id=1.0.4

udisks v2: http://cgit.freedesktop.org/udisks/tree/src/udiskslinuxdrive.c?id=2.1.2#n1195

As you can see, both v1 and v2 does this by writing a '1' to the 'remove' sysfs attribute on the parent USB device, as per

 http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=253e05724f9230910344357b1142ad8642ff9f5a

and this makes most USB-attached disk drives actually power down - at least all the different devices that I've tested with.

However, what's missing in v2 (and present in v1) is the following steps

 1. sending the SCSI SYNCHRONIZE CACHE command
 2. sending START/STOP UNIT command
 3. unbinding the USB Mass Storage kernel driver

Notably, there's actually a TODO item in v2 for doing this:

 /* TODO: Send the eject? Send SCSI START STOP UNIT? */

Now, I don't think that 3. is necessary as it happens as part of writing to the 'remove' sysfs file. That leaves 1. and 2.

Here's what I'd like to you try. Does

 sg_start --stop /dev/sdX

do what you want? If so, we should add 1. and 2. to v2.