I haven't confirmed your test, but you might try a UDF that does something similar.

Diskspace()
http://www.kixtart.org/forums/ubbthreads.php?ubb=showflat&Number=82589

How to use UDFs -
http://www.kixtart.org/forums/ubbthreads.php?ubb=showflat&Number=81943#Post81943

The rest of the UDFs are here -
http://www.kixtart.org/forums/ubbthreads.php?ubb=postlist&Board=7&page=1

Kixtart is still supported, and used by at least a few more soles like yourself. If we confirm this bug we'll need to pass this along to Ruud and hope he will include the fix in the next release.
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(... better days ahead)