The disk space monitor plasmoid is empty and shows no available mountpoints in its settings dialogue.
KDE 4.2.98 (KDE 4.3 RC3) with Qt 4.5.2 on openSUSE 11.1 64
What do you mean by “available mount points”. Every directory can serve as a mount point.
Maybe that question would be answered if I knew what your disk space monitor plasmoid is for, but the term “available mount points” draw my attention and curiousity.