Issues trying to boot macOS Mojave off bootable external USB

wildtree

macrumors newbie
Original poster
Jul 1, 2019
3
0
I'm trying to run macOS off a USB 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?
 

benshive

macrumors 6502
Feb 26, 2017
275
2,092
United States
Boot into internet recovery (CMD + R on startup) and wipe the drive using Disk Utility there. After it's wiped you can also install the version of macOS that shipped with your Mac on the drive and then update it to Mojave.
 

Sylverton76

macrumors newbie
Jun 27, 2019
22
24
UK
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.
Am I correct in assuming that the USB stick is formatted incorrectly and not the HDD? If so, simply format the stick again and recreate the installer. In Disk Utility, select the tab on the top left marked view and change to 'Show All Devices'. When you choose the top level i.e. the USB stick rather than the volume, you'll now see a scheme box. Select GUID along with Mac OS Extended (Journaled) for the format option.
 
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jbarley

macrumors 68040
Jul 1, 2006
3,769
1,614
Vancouver Island
Boot into internet recovery (CMD + R on startup) and wipe the drive using Disk Utility there. After it's wiped you can also install the version of macOS that shipped with your Mac on the drive and then update it to Mojave.
Last time I checked, Internet Recovery was "opt+cmd+R", did Apple change something when I wasn't paying attention?;)
 

wildtree

macrumors newbie
Original poster
Jul 1, 2019
3
0
Am I correct in assuming that the USB stick is formatted incorrectly and not the HDD? If so, simply format the stick again and recreate the installer. In Disk Utility, select the tab on the top left marked view and change to 'Show All Devices'. When you choose the top level i.e. the USB stick rather than the volume, you'll now see a scheme box. Select GUID along with Mac OS Extended (Journaled) for the format option.
Hey! thanks for your reply - That's possibly correct. I just went through and reformatted as you mentioned however I'm getting a "this disk is locked" message when trying to install onto the usb

For more context I'm trying to see if there's an issue with my current os and have been told to "leave your current Mac OS install on the laptop. And install a fresh copy of macOS on the external flash drive or SSD. Once you boot into the fresh OS" - am I doing the right thing?
 

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Sylverton76

macrumors newbie
Jun 27, 2019
22
24
UK
Hey! thanks for your reply - That's possibly correct. I just went through and reformatted as you mentioned however I'm getting a "this disk is locked" message when trying to install onto the usb
It is locked because you are trying to install Mojave on the USB partition with the installation files. You need a second USB formatted drive to install the OS.

For the person above, Apple states internet recovery is as jbarley mentioned: Option-Command-R or Shift-Option-Command-R at startup. Command-R boots the built-in recovery partition from the boot drive...unless this partition is missing.
 
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LuisN

macrumors regular
Mar 30, 2013
199
184
Torres Vedras, Portugal
Sorry, misread your comment.
Friends again? ;-)
[doublepost=1562167316][/doublepost]
It is locked because you are trying to install Mojave on the USB partition with the installation files. You need a second USB formatted drive to install the OS.

For the person above, Apple states internet recovery is as jbarley mentioned: Option-Command-R or Shift-Option-Command-R at startup. Command-R boots the built-in recovery partition from the boot drive...unless this partition is missing.
You're absolutely right, my error.
 

wildtree

macrumors newbie
Original poster
Jul 1, 2019
3
0
It is locked because you are trying to install Mojave on the USB partition with the installation files. You need a second USB formatted drive to install the OS.
Okay cool so to confirm I need 2 USB's in the Mac to test a newly loaded OS without touching the existing one/HHD - one loaded with the OS installer (that I boot off when starting the mac ) and 1 to install it onto?
 

Sylverton76

macrumors newbie
Jun 27, 2019
22
24
UK
Okay cool so to confirm I need 2 USB's in the Mac to test a newly loaded OS without touching the existing one/HHD - one loaded with the OS installer (that I boot off when starting the mac ) and 1 to install it onto?
That's correct. One of the USBs will contain the installation files, the second is where you install the OS. The internal drive remains intact and not affected by this process. Simply remove the drives when you are done testing and the machine will revert to booting the internal drive.

FYI, if you do struggle to boot the internal drive once you are done, boot holding down the option key and select the internal drive. When booted into the OS, open up System Preferences > Startup Disk and make sure the internal drive is selected as the startup disk.
 

Fishrrman

macrumors P6
Feb 20, 2009
17,152
5,516
The OP wrote:
"I'm trying to run macOS off a USB so I can troubleshoot some issues I've been having with a monitor."

Before going further, how about describing "the issues" you're having?
What kind of Mac do you have?
What year was it made?
What OS is currently running on it?
If it's an external monitor, what port is it connected to?
 
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