I came across this thread while googling for a solution for the similar problem. I was hitting on the same error while trying to mount my local windows partition while running Debian.
Though I did not find a lead here I discovered later that my ntfs-3g binary no longer had it's suid bit set. Resetting it's suid bit restored the behavior of being able to local partitions without being root.
$ chmod 4755 /usr/bin/ntfs-3g
I thought that might help you in case you are still practicing using 'sudo' while mounting. Also I think the solution might be the same even though you use Slackware. Hope this helps
My actual problem is now to discover how and why the 'ntfs-3g' binary's suid bit keeps resetting itself every time I boot.
That the suid bit has to set so that unprivileged users can mount was learnt from "How to mount local ntfs partitions without being root