Ran into a serious bug and reported it...
Debian SID (VSIDO) 4.3.10 VirtualBox
The symlink for mount.vsbox symlink is broken (missing) in 4.3.10 Guest Additions update
/sbin/mount.vsbox is/was a symlink to usr/lib/VBoxGuestAdditions/mount.vsbox
In order to get mount.vsbox and my mounts working again I had to manually setup a symlink of /sbin/mount.vsbox to /opt/VBoxGuestAdditions/mount.vsbox
Once done, I could mount again
Bug Reported Here (https://www.virtualbox.org/ticket/12889)