I wanted to spin down a hard drive before disconnecting. I have an external hard drive and even after I unmounted the drive the disks were still spinning.

Find the device label of the mounted drive:

$ lsblk
NAME   MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda    253:0    0  20G  0 disk
└─sda1 253:1    0  20G  0 part /
sdb    253:0    0  10G  0 disk
└─sdb1 253:1    0  10G  0 part /media/drive/

Running mount with no arguments will also help to find the device label

To umount the partitions on the drive:

udisks --unmount /dev/sdb1
note:It is extremely important to ensure that you are unmounting the correct device.

Finally, use udisks to stop the drive:

udisks --detach /dev/sdb

udisk can be installed on Debian systems using apt-get:

sudo apt-get install udisks
source:http://askubuntu.com/questions/56270/how-can-i-spin-down-external-hard-drive