Sometimes my system freezes when I use my external USB HDD (Western Digital 500GB). I am using a freshly upgraded Ubuntu 11.10 installation.
The freeze is not reproducable but always happens right after I unplug the device. The last time it happened right after I clicked the "Eject" Button in the Gnome Shell notification that appears when you plug in an external drive.
It is a real freeze, I have to hard reset the system when it happens.