Whoops. I have mixed up my Windows licenses. I have an OEM Windows 7 license, upgraded to Windows 10 for free. I have two full Windows 10 Retail licenses, one a physical copy and the second a digital download. I have one MacBook, one iMac and one PC. For various reasons I'm always removing and re-installing my Boot Camp partitions. I have three license key numbers written down but now I can't remember which I installed on what computer (Boot Camp is not currently installed so I can't retrieve the key directly from Windows even if that is possible). Does anybody know a good way to work out which key belongs to what device? This time I'll make a record of it.