Virtualbox – FATAL: Could not read from the boot medium!

I use the very handy free virtualisation software Virtualbox and usually it’s very well behaved. However, when I tried to start a Windows 8.1 virtual machine this evening, I immediately got the message: FATAL: Could not read from the boot medium! System halted. The memory and CPU settings for the VM also seemed to be wrong so I was guessing some kind of corruption. Not the end of the world in this particular case but still a pain. If you Google that message, you mostly find forum posts from people who’ve just started using Virtualbox and don’t know you need to mount an ISO in order to boot a brand new VM and install the operating system. But it’s also what you get if Virtualbox looks at your VM’s virtual disk and sees nothing that looks like an OS. I had a recent backup of the VM but mounting that got the same result. But I did notice that as well as the normal Virtualbox configuration file my-vm.vbox, there was a second one dated slightly earlier called my-vm-1.14-windows.vbox. I tried mounting that instead and it worked! The memory and CPU settings were also back to the expected values. I’m guessing that if I looked more closely, the hard disk settings in the apparently corrupt config file were also wrong and that’s why it couldn’t boot.