When you lost connection to a machine that is still locked to the dead session
I've opened a remote ssh session with X forwarding and opened a virtual machine with VBoxManage::
$ VBoxManage startvm 'Debian Master'
After some time, the dynamic IP address of the host machine changed, then I lost the connection to the virtual machine. I reconnected to the host again and tried to start it again, but received an error::
$ VBoxManage startvm 'Debian Master' VBoxHeadless: error: The machine 'Debian Master' is already locked for a session (or being unlocked) VBoxHeadless: error: Details: code VBOX_E_INVALID_OBJECT_STATE (0x80bb0007), component Machine, interface IMachine, callee nsISupports Context: "LockMachine(session, LockType_Write)" at line 817 of file VBoxHeadless.cpp Trying to stop it hanged the command. So, I had to search for a process that still had the control over the virtual machine:: $ ps ax | grep -i virtualbox 2067 ? S 9:23 /usr/lib/virtualbox/VBoxXPCOMIPCD 2072 ? Sl 29:57 /usr/lib/virtualbox/VBoxSVC --auto-shutdown 14215 ? Sl 0:40 /usr/lib/virtualbox/VirtualBox /vms/node002.ova 18722 ? SLl 78:13 /usr/lib/virtualbox/VirtualBox --comment Windows --startvm 0ac2589a-cdfd-424a-b908-63512238f7d9 --no-startvm-errormsgbox 27227 ? Sl 2:07 /usr/lib/virtualbox/VirtualBox --comment Debian Master --startvm efc1d9f1-2aec-4fbe-aaa3-fd62ca578ab0 --no-startvm-errormsgbox 27983 pts/3 S+ 0:00 grep -i virtualbox
See that it is the process 27227. After killing it I could start the virtual machine again.