Further to this LQ thread
I have investigated further and found a persistent "history effect". AFAIK this is specific to Slackware hence posting in this sub-forum rather than in the Virtualization forum.
After a default installation (Slackware64 13.1 and VirtualBox 3.2.8 but has been seen with earlier versions) the VM's host window's Devices -> USB Devices list is greyed out.
The problem can readily be fixed by adding a usbfs line to fstab but it should not be necessary; udev should do the job. And udev does do the job if /etc/udev/rules.d/10-vboxdrv.rules is renamed as 91-vboxdrv.rules (that moves it to the end of the lexical ordering) and the fstab line is removed and the host is rebooted.
This solution suggests that some inter-action amongst the udev rules is causing the problem. The problem rule(s) could be identified by moving <nn>-vboxdrv.rules within the lexical ordering until shifting it just one position triggers the problem.
Now for the persistent history effect. After renaming to trigger failure (say 10-vboxdrv.rules) and then renaming to a previously working name (say 55-vboxdrv.rules) it still fails, even after reboot! The persistent history effect is removed by restoring the fstab line, rebooting, removing the fstab line and rebooting.
Simulating reboot effects by re-initialising udev rules (udevadm control --reload-rules), usbfs (forgot to umount, ran commands from rc.S to mount) and VirtualBox (/etc/rc.d/rc.vboxdrv restart) did not remove the history effect but a full reboot did.
I have no idea what file change(s) are storing the history effect.
Mostly I'm publishing this in case it helps anybody or in case someone can suggest ways of removing the history effect without rebooting. At the current rate of progress it's going to take a long time for me to home in on the problem rule and maybe find a fix.
Meanwhile there's an effective workaround.