This bug was reported on Ubuntu Launchpad as: https://bugs.launchpad.net/bugs/415037 From the launchpad report: There's two mounted and two unmounted removable filesystems in this screenshot. Would you have been able to tell them apart at a glimpse? Please make the two icons more distinct. http://launchpadlibrarian.net/30429793/Screenshot.png
The original ubuntu bug was closed because there is now (thunar 1.2.x) an eject icon on the right side, which makes easier to distinguish that.