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~ sudo dpkg-reconfigure virtualbox-dkms gives no errors, ~ sudo apt-get install linux-headers-`uname -r` confirms current headers are the latest, ~ confirmed that I'm a member of the vboxusers group,
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I thought at first the problem may be caused by Ubuntu changing to systemd from init so I removed systemd. This partition runs VirtualBox 4.3.26 which was upgraded from 4.3.10 through the normal dist-upgrade process from standard repos. At a guess I'd say the crash comes just as the Windows login screen is about to appear. I rsync this partition into a new partition and commence upgrading Ubuntu towards the next LTS currently I'm on 15.04 but all the VMs crash on start after a couple of moments. I have several partitions on my system with one being my "production" environment comprising Ubuntu 14.04LTS 64bit with VirtualBox 4.3.10 running Win 7 Pro (64 bit) and Win XP (32 bit) VMs stored on a separate data drive. I'm very new to tech aspects of VirtualBox although I have been using it for a number of years (brilliant product).
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