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I'm trying to run macOS off a USB drive, so I can troubleshoot some issues I've been having with a monitor. I've followed this how-to video, and can reboot (holding Option + Power On). Once I choose the USB I try to install macOS Mojave, however it stops me when I try to select the greyed out USB.

I get the following message:

This disk doesn't use the GUID partition scheme. Use Disk Utility to change the partition. Select the device containing the disk, click the Erase button, select the Volume Scheme and then click Erase

I follow the prompts and when I go to erase it doesn't allow me to do it as it's the usb device itself I've booted/started the computer with.

How can I fix this? Have I done something wrong along the way?

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You will need to reformat the card in the Mac, it will by default suggest GUID/HFS+.

Most cards, as supplied, will be formatted MBR not GUID, so if you just add an HFS+ partition, the Mac cannot boot from it, no matter if you did the rest of the process correctly.

& as Nimesh says - you cannot format the drive you are booted from [I'd missed that bit]

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To run macOS off a USB drive, it needs to be installed on it first.

Unlike some Linux distributions, you cannot run macOS in live mode from the bootable installer USB. Also, it appears that you are trying to install macOS onto the same USB stick that you booted off. This is clearly not possible to do.

You can, however, connect a separate external USB stick/drive and install macOS onto it. Once installed, remove the USB stick containing the installer, start your Mac, press and hold Option key and boot off the external disk onto which you installed macOS.

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