NCDU is a ncurses version of Linux "du" (disk usage) tool. It analyzes your hard disk and shows the amount of disk space every folder uses.
This way it's very simple to find those folders with the largest files. You can even delete files and folders from within ncdu.
After installing ncurses as described at http://www.welped.com/2007/09/03/an-easier-editor-option-for-mybook/ you can easily install "ncdu" on your mybook.
Get ncdu from http://dev.yorhel.nl/ncdu/
tar xzvf ncdu-1.3.tar.gz
Have fun! :-)