Connect it to your computer and launch Disk Utility.

Eject the volume by clicking the Eject button alongside it. Now select the device name. Go to Erase and type a name for the drive. Now click Erase. Remember that this will wipe everything on your drive. Now download the version of macOS you want to use.

Why external hard drives greyed out in Disk Utility?

Double-click this to get started. Click through the license agreement. Click Show All Disks and select your external drive. All the necessary files will copy over to your drive first, which might take up to 10 minutes. Then your Mac will shut down and the full installation will start.

Why a flash drive is not showing up on Mac?

This takes a while, and the time estimate is not necessarily accurate. In our case the estimate was 15 minutes, but the process was still chugging away after half an hour. Faster hardware is definitely a plus here. Now you just need to set it up as you normally would. Next time you start your computer, it might revert back to booting from your internal hard drive.

Install, Boot, and Run Mac OS X From an External Hard Drive

Click the lock and enter your password to change the settings. Select your USB drive from the list and click Restart. The second method is to turn on your computer and hold down the Option key. Choose the external drive and hit Enter to continue booting.

Both of these methods set the external drive as the default. Therefore you can simply unplug it whenever you want to boot to your internal drive instead. When running macOS off an external drive, you still have to shut it down in the normal way. This could cause you to corrupt your data. You need a full computer reboot to switch from one drive to the other.

The entire process of setting your Mac to boot from USB should take around half an hour at most. Or you could install macOS on a flash drive, stick it in a drawer, and keep it for emergencies.


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Booting your Mac from a USB drive gives you a way to fix any boot problems your computer experiences. The Mac OS startup manager will appear, allowing you to select the device to boot from. To start up in Safe Mode , hold down the shift key and then start your Mac. It will also delete some of the startup caches that may also be preventing your Mac from starting successfully.

When First Aid is finished, restart your Mac normally.

What we are doing.

You should use this startup mode only for troubleshooting and not for running day-to-day applications. Press return or enter after you type the above line. Fsck will start and display status messages about your startup disk. When it finally finishes this can take a while , you will see one of two messages. The first indicates that no problems were found. The second message indicates that problems were encountered and fsck attempted to correct the errors on your hard drive.

If you see the second message, you should repeat the fsck command again. Share Pin Email. Tom Nelson has written hundreds of articles, tutorials, and reviews for Other World Computing and About.

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He is the president of Coyote Moon, Inc.