A simple solution to the Hyper-V error “Virtual machine ‘NN’ failed to change state”: check your disk space

I was earlier this week helping a customer to set up a virtual environment, but in their current Hyper-V environment, there was a problem: we couldn't start our virtual machine. Instead, Hyper-V showed the following error message while pressing the Start button of the virtual machine:

The application encountered an error while attempting to change the state of 'NN'.
'NN' failed to change state.

While there might be other reasons for this error, in this particular case, the solution was very simple: there's wasn't enough free disk space to fire up the virtual machine. When you start a virtual machine, you need at least an equal amount of free disk space than there is configured memory for the virtual machine. If this fails, Hyper-V isn't able to display a very exact error message. This can make troubleshooting a little hard.

The moral of the story: sometimes, technical problems are caused by the basics. Remember to check these, too!