What do you have for the
ioBvar(arealist) setting? Each area is a root directory in your FTP.
For example, if you had the arealist set to "APPS MP3 REQS", ioBanana would check the free space in /APPS, /MP3, and /REQS. Directory paths in ioFTPD are case sensitive - if the root directory is /apps and you have APPS in the arealist, this will not work. In which case, you would have to put "apps" in the arealist.
Hope this helps.