I made some changes in this plugin to it work on FreeBSD. Maybe you would like to merge this changes in the main source tree. You can get the patches here: https://github.com/daniloegea/xfce4-diskperf-plugin/tree/master/files
Thanks, merged in http://git.xfce.org/panel-plugins/xfce4-diskperf-plugin/commit/?id=0faba9adc585a02071b4ab1d8761413b4cc35a9a
aaand.. fixing