Civilians should create a bootable installer on a USB thumb drive, of course: https://support.apple.com/en-us/HT201372 https://support.apple.com/en-us/HT201372

Civilians should create a bootable installer on a USB thumb drive, of course: https://support.apple.com/en-us/HT201372 https://support.apple.com/en-us/HT201372 And I'm only being slightly facetious. The ability of modern hardware to download install an OS directly from firmware is a truly great and useful modern innovation, but it leaves a lot of unanswered questions: What if my network is bad? What if this used hardware is stolen? What if I don't have an account? What if Apple goes bankrupt? USB/SD/DVD installers do not require network access nor any sort of account/purchase verification, and can be created (relatively) easily and officially from downloaded installers. I have bootable disk images for every OS X installer since OS X Mavericks and I highly recommend it. I'll include a USB installer whenever I sell an old laptop. It costs me next to nothing and gives the buyer some reassurance that they're not buying a brick. And if I decide to hang on to a piece of hardware, I have some assurance that I'll be able to use it 10 years from now.