Just to be clear: 1) I haven't changed any hardware on this machine. I don't mean recently, I mean ever. 2) This has happened before and used to be quite a common problem. I just forgot how I resolved it and now all i can find is how to solve this happening due to hardware changes. In "About this Mac" on a MacbookAir 4,2 (Mid-2011) running OSX Sierra 10.12.4 my serial number has gone bye-bye. It is listed as "Unavailable". Under System Report it is "Not Available". If I have to use a Serialiser to fix this (something I don't recall doing in the past, I'm fairly sure I did it from Terminal or from booting to CLi mode) then so be it, but I'd love some information on which Serialiser and from whence to get it that's newer than the days of Mountain Lion. For those unfamiliar: It's something that goes weird after some OS upgrades/updates. Apparently nobody knows why. It's just a pain, and while iCloud and even software updates continue to go along blissfully unaware, they start to behave in peculiar fashions. I.e. you might have iCloud Keychain spontaneously turn off, try 50 times to enable it, get prompted 75 times to enter your iCloud login information, give up, and then next time you use the computer Keychain is turned on again (that last step is filed under "If you're marginally lucky") So, yeah, that's where I'm at. Google is being … well … Google. DuckDuckGo at least gave me results that matched my problem but no one had given them an actual answer to their problem (one reply was literally "I haven't swapped hardware" "So, sounds like you had your logic board replaced!") and I can't find any of the older discussions I used the last time this happened. Both to the machine in question and to a 2004 Powerbook.