There are a few options, depending on your firmware version.
1. Go into the web interface, click on "Maintenance". There should be a "Shutdown" option.
2. Older units have a small "Power" button on the back.
3. Newer units have a small switch with On, Off, and Auto as option. Swtiching it to off will power it down, but it may take a minute or two for it to cycle through.
As Sethz said, the lever on the back should have an off option on recent LS Pro Duo's.
check: http://cdn.cloudfiles.mosso.com/c85091/LinkStation_Pro_Duo_Manual_web.pdf
and check page 8, titled "Power Mode". Should explain it quite well for you.
Hope this helps.
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