I have an usb external disk and I spin-down it through "hd-idle", a utility I found here: http://hd-idle.sourceforge.net.
I thought that when Ubuntu shutdown, it unmount all partitions, whether these belong to internal or external hard drives.
I noticed, instead, that if an internal drive is in standby, this is awakened at the time of the system shutdown (probably just to unmount any partitions mounted on that disk even if it does so when there are no partitions mounted, and in fact in my opinion, it would also be preferable if in this case it remain off) but the same doesn't occur for my external hard-drive.
This let me think that "external" partitions are not correctly unmounted. Anyone knows why this happens?