My Linux host os disk partition filled up due to .xsession_errors consuming all available space and subsequently the running VBox VMs did not shut down cleanly and would not restart. Below is a link indicating possible fixes for .xsession_errors issue. http://askubuntu.com/questions/177058/xsession-errors-file-is-huge-how-can-i-disable Both the active and previous versions of the vmName.vbox XML files became corrupted and the StorageControllers element contents were missing. I tried adding them from an old backup, but that didn't work since I had moved and rebuilt the VM. Should have had a more recent backup. I was able to recover by remapping the storage in the VirtualBox Manager, but it took several tries until I discovered the correct storage type and attributes. If I had recorded those somewhere, it would have been easier to recover.