Safe Mode disables login items and nonessential kernel extensions. Hold the Shift key during startup to boot your computer in Safe Mode.The startup manager searches all the volumes connected to your Mac and displays those that have a bootable operating system. Hold the Option key during startup to open the Mac OS startup manager, allowing you to select a disk to boot from.Hold the Option + 'd' key during startup to boot up using the AHT over the internet or Apple Diagnostics over the internet.Hold the 'd' key during startup to boot up using the Apple Hardware Test (AHT) or Apple Diagnostics.This mode lets you use any Mac with a FireWire or Thunderbolt port as the source for your boot-up system. Hold the 't' key during startup to boot in Target Disk Mode.Hold the Option + 'n' key to boot from the NetBoot default startup volume.NetBoot volumes can be created with OS X or macOS Server, allowing you to boot from, install the Mac OS, or restore the Mac OS from the server on your local network. Hold the 'n' key during startup to boot from a networked computer that has a NetBoot volume.If you have created a bootable Mac OS installer on a flash drive, this is an easy way to boot from the installer. Hold the 'c' key during startup to boot from a bootable CD, DVD, or USB flash drive.In some cases, an alternate OS may prevent the Mac's normal boot manager from running. You may find this useful if you have your Mac set to boot to a non-Mac OS volume, such as Windows or Linux.

Hold the 'x' key during startup to force the Mac to boot from OS X or macOS, no matter which disk is specified as the startup disk.

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