I recently filed a bug report against nautilus because I was "Unable to unmount
the selected volume."  Someone commented that this may be because an application
(gnome-terminal, gedit) was using the file system and therefor it couldn't
umount the system.

This got me to thinking that if instead of simply reporting that the "device is
busy", it would be brilliant if nautilus indicated what applications were
blocking   the umount.

As a start, this could be a simple as indicating what applications are blocking
the umount (maybe using 'fuser'), and possibly indicating if another user was
blocking the umount (so that you could ask them to get out if possible).

A better step might be to show which windows are the offending windows where
possible by highlighting them in the workspace switcher (or by highlighting the
title bar using a flashing or another color).  This might help where the
offending application is something like gnome-terminal which is often run a
number of times.

In cases where the applications isn't running as a window on the desktop, it
might necessary to fall back to displaying a list of (non-windowed) applications
in the dialog

It would definitely be a good step forward in helping users assess what's
causing the problem.  After all, the comment "umount: <device>: device is busy"
isn't that informative.