Diconnecting USB Devices from within a script

Is there a way of Diconnecting a USB device (like a external hard drive)
from a command inside of a
script?

I am trying to set up a backup solution using USB external hard drives, and
would like to add a line at the end of the nightly script that disconnects
the device so that the drive can be switched in the morning without the user
having to have access to the server console.

Thanks in advance for any help.
Please CC any advice to my email at (E-Mail Removed)
Randy