This is probably a combination of OpenSuSe and VirtualBox ....

Environment:

Host: OpenSuse 12.3, AMD FX quad core 16GB, VirtualBox 4.2.4
Guest: OpenSuSe 13.1 KDE desktop, fresh build Thurs 09-Jan-2014, updated to latest patches at that time. Allocated 2 cores, 2048MB memory, 128MB video memory. The guest has one virtual disk, allocated thus:
-- /swap: 4.00 GB
-- /: 25.00GB
-- /home: 11.00GB

The hardware is not being strained.

On the GUEST:

If a user other than root logs in, the system shutdown hangs with a message: Partition /home still busy; giving up!

Nothing appears in the log files; it happens after rsyslod has stopped.

The guest will not respond to any commands and requires that the process on the host be sent a TERM signal. Very crude.

Note that this behavior does not occur if only root has logged in since last boot! It matters not whether the non-root user was first, last or in between any root logins.

As root, checking for open files on /home after the non-root user logs out reveals:

fuser -m /home
(pid-of-offending-process)
ps aux | grep (pid-of-offending-process)
(user) [... snip ...] /usr/bin/VBoxClient --draganddrop

But (user) has logged out?

Logged in as root, I tried fuser -km /home and that resulted in the shutdown still hanging, and one of the host's cpu cores being driven to 100%. The VirtualBox host process showed in System Monitor as 25% of the cpu (1 core of 4 == 25%). Sending that process SIGTERM sets the VM's status to "Aborted" in the VBox management GUI. Fortunately, thusfar the shutdown seems to have run to such a point that crashing the VM doesn't leave things dangling; the reboots have been clean. But I am not happy ....

This behavior is specific to OpenSuSe 13.1 as a guest. The same host has other VM's that work perfectly well (business-critical WindowsXP and Windows 7, CentOS 6.5, Open SuSe 12.1) which is why the version of VirtualBox is 4.2.4 -- IT WORKS, except with the latest OpenSuSe. Note that the version of the VBox Guest Additions in the 13.1 guest is from the 13.1 distro/repos:

(As reported by SoftwareManager)

virtualbox-guest-kmp-desktop 4.2.18_k3,11.6_4-2.5.1
virtualbox-guest-kmp-tools 4.2.18-2.5.1
virtualbox-guest-kmp-X11 4.2.18-2.5.1

What is preventing the OS from clearing this process and successfully unmounting /home?