If you try to unmount a busy device you will get an error message. However, that error message is completely useless to people without a specific knowledge, i.e. knowing how to use the 'lsof' command.
Ideally a dialogue would show up showing what processes are preventing you from unmounting it, including a "Force" button and PolicyKit integration. Maybe the processes currently working on the data on the device could be highlighted to prevent painful mistakes

However, this is a bit of work, so a temporary solution could be to make the error message clearer, e.g. "The device you're trying to unmount is currently still in use by one or more applications. Please close those first."

This bug was originally raised by Mr. Mizzen on Launchpad in Ubuntu at <https://launchpad.net/bugs/486024>.