Since Apple released the new Airport Utility with the simplified UI, there doesn't seem to be an option to "Disconnect All Users" to kick all users off a hard drive to safely remove it when connected to an Airport Extreme. See the screenshot for this button in the previous version of Airport Utility.
How should I do this in the new version?
Best Answer
Consensus is that Apple simply forgot it - or didn't make the UI clear!
Deselecting "Enable file sharing" looks to have replaced it. At the very least that will disconnect users before you pull the plug.