Some do, some don't which will make answering how a little tough, but "how?" is that the software on the drive recieved a spindown command.
Bob
i have an external had that i "made" (i got a nexstar 2 enclosure and a 60gb seagate drive) i was actually wondering how the drive spins down. when i am done using it i click the safely remove button and remove the usb cable then unplug and power off the drive. does the drive spin down when you "safely remove" the drive? or does it matter?

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