In device_removed, you emit the 'device-added' signal, which causes thunar-volman to try to mount the device after it gets removed. Oops. Should emit 'device-removed' instead, I assume.
Created attachment 1125 thunar-vfs-fix-signal-typo.diff
Committed with revision 25691. 2007-05-09 Benedikt Meurer <benny@xfce.org> * thunar-vfs/thunar-vfs-volume-hal.c (thunar_vfs_volume_manager_hal_device_removed): Emit the correct signal when removing devices. Patch from Brian Tarricone. Bug #3205.