I took the slackware64-current-26_Mar_2013-DVD.iso from slackware.no
and did a quick and dirty full (terse) install from it. The "mount -oloop / umount" problem was not present.
I then set the slackpkg mirror to slackware.org.uk
and did an upgrade-all. The "mount -oloop / umount" problem was still not present.
I then installpkg'ed the salix/mint packages (see my previous post) and then the "mount -oloop / umount" problem popped up !!!
As an experiment I closed caja (the mate file manager, "nautilus") and the problem was gone. Started caja and the problem was there again !!!
This outcome puts me in a quandary: my workflow is greatly enhanced by a treeview file manager (nautilus, konqueror, dolphin). I never could become friends with KDE4 so I am "stuck" with nautilus and friends.
Luckily the "mount -oloop / umount" problem is not fatal: the [u]mount seem to have completed when the error popup comes. I can tolerate things for the time being.