Boot into the Recovery partition, or from a local Time Machine backup volume ( option key at startup.) When the OS X Utilities screen appears, launch Disk Utility and follow the instructions in this support article, under “Instructions for backing up to an external hard disk via Disk Utility.”ī.

You need an external hard drive to hold the backup data.Ī. There are several ways to back up a Mac that is unable to boot. If you don't care about the data that has changed since your last backup, you can skip this step. If you want to preserve the contents of the startup drive, and you don't already have at least one current backup, you must try to back up now, before you do anything else. This Apple developer document is a good place to start.The first step in dealing with a boot failure is to secure your data. Should you be curious about the technical details of the boot process,

System Management Controller (SMC), & so on are recommended procedures for this type of problem. What can affect the Startup Manager is corrupted PRAM or fundamental hardware problems, like power management issues. The storage media supplies the boot loader code that actually loads the OS, so drive problems can prevent the OS from loading or its partition from showing up in the Startup Manager, but no problem on one drive should prevent other boot sources from displaying their icons if they contain an OS capable of booting that Mac. (This is the main reason for its primitive user interface.) It is a part of the Mac's hardware, not something loaded from a hard drive or other storage media. This should never happen because of a corrupted drive: the BootROM of the Mac is responsible for selecting which operating system to run from some partition. A severely corrupted drive could make this process fail, and not allow you to boot from any drive.

Starting with Option pressed looks on each drive for a bootable system, so it knows which drives icons to display.

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