Just a few comments on my progress. As it appears there is no way to get access to a volume unless one has administrator rights (or – presumably – the volume ACL was changed?). That, of course, creates the problem that the speedup is only available to admins.
However, I think that WinDirStat is mostly used for administrative purposes anyway, thus it will be fine to limit the speedup to this part. Please note that this does not mean that running it as unprivileged user wouldn’t work anymore, but even now this yields biased results …
BTW: the speedup is quite astonishing in my experiments so far.
// Oliver
PS: Yes, I’m talking about read-only access to the volume …