So I've followed the apples guide and also another guide on here on creating a bootable drive for MacOS Mojave.

I bought a new flash drive, plugged it in and erased it.

  1. I set the SanDisk Cruzer Blade Media to a GUID Partition map
  2. I then created the volume and set it as MAC OS Extended (Journal)
  3. I run the commands in terminal and it copies the Mojave over fine, says complete.
  4. I restart my Mac and select the Install Mojave drive (Not my MACINTOSH HD)
  5. In disk utility I select to install Mojave
  6. Click continue on the installation
  7. Then all my drives appear, but my bootable drive which is called Install MacOS Mojave says "This disk is locked"
  8. I've tried to erase this in the disk utility mode and it says failed to unmount.

I'm really not sure why it's locked and this is the problem I'm facing, how can I get this unlocked? All the videos I watch nobody has this error except from myself.

SanDisk Cruzer

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  • It is hard to tell if you are trying to select the installer as the destination in step 7 & 8. Are you able to erase your MacOS volume for a clean erase install or do you lack a backup? Renaming the volume you target might help clarify whether this is a problem with your procedure or a problem with your hardware. Details like this may help or linking to the Apple guide you followed.
    – bmike
    Commented Apr 30, 2020 at 10:05
  • Thank you for your response bmike. I definitely am selecting the installer as the destination, when I start up the mac I have 2 options, 1 being the macintosh HD and the other being the bootable drive, I choose that one, then click install, click continue and agree to the terms and conditions, then I have to choose which drive I want to install MacOs Mojave on, I try to choose my flash drive (Install MacOS Mojave) but it says "This disc is locked" I then try to erase the disc(to see if it fixes it) inside disc utilties and it says it fails to unmount, but I can erase it fine when I format it
    – Lee Howard
    Commented Apr 30, 2020 at 10:07
  • You can’t install onto an installer. I’ll put a proper answer to say that. If you want to install macOS you need two external driver or skip the bootable installer, just wipe the external device and run the installer from Macos. Basically, don’t copy the installer to the disk you want to target for the install.
    – bmike
    Commented Apr 30, 2020 at 10:15
  • Also, you’re quite far down a path you might not want to travel, I would make an entirely new post. Show your About this Mac storage size from your Catalina system and ask if there is enough space to make a second APFS container so you can dual boot Catalina and Mojave on your main fast storage. That’s the best solution here, not putting Mojave on a slow external drive.
    – bmike
    Commented Apr 30, 2020 at 10:34
  • 1
    Yeah I think that's honestly what I'm trying to do. This is the other guide apple.stackexchange.com/questions/374846/… - I followed this, but what I now realise I wasn't selecting the "Mojave" partition, probably the most important part, lol. I've got work in 20 minutes, so once thats over I will give this another go! This will enable me to dual boot correct?
    – Lee Howard
    Commented Apr 30, 2020 at 10:42

1 Answer 1


You must select a drive that’s not the installer in step 7 since the volume where the installer exists is read only until the install completes.

I’ll assume this is your Apple guide.

Any drive except the installer you prepared should be chosen. This lock is to force you to it over write the installer.

  • if you want to erase the main drive - https://support.apple.com/en-us/HT208496
  • If you have room, make a second volume on APFS beside the Catalina so you can put Mojave not on your main drive and not on the main external partition.

This is something that trips up people that come from Knowing how to install other OS. Use internet recovery if you have no bootable macOS to run an installer rather than copying your installer onto the volume you want actual macOS on.

Also, since you may not wish to erase your Mac (it’s running Catalina and you want Mojave on an external drive) and if there’s not enough free space on the internal or external for a second volume to hold Mojave, get a third external drive so the installer can be on one external and boot to it to install to the second external drive (or second volume).

  • Thank you again, but this is the guide I was looking at support.apple.com/en-us/HT201372 I'm not trying to reinstall, I want to actually install Mojave on my flash drive, but it's saying it's locked. I don't want to wipe my Mac. I simply want to have Catalina on my main drive, and Mojave on my usb, I want Mojave on my USB so I can play 32bit games. I hope this clarifies what I want
    – Lee Howard
    Commented Apr 30, 2020 at 10:19
  • Bingo @LeeHoward. You can’t use the flash disk to host the installer and be the destination. Just erase the flash disk, run the installer from any other volume. If you can’t run the Mojave installer from Catalina, you need a second flash drive. One to be blank and not to run the Mojave installer. Maybe you can partition your flash into two volumes but I get inpatient reading from one flash and writing to that same flash. Might take hours to complete.
    – bmike
    Commented Apr 30, 2020 at 10:22
  • Ahh so just to clarify and make sure I get it. 1. restart the mac, when I see the 2 drives I choose Install mac os mojave, click install. 2. THEN when it shows me the drives to install it on, I just choose my MAIN Macintosh HD, correct? 3. This WILL NOT, erase my mac? correct?
    – Lee Howard
    Commented Apr 30, 2020 at 10:27
  • That will fail too, see my edit. Mojave won’t install on top of Catalina. The guide I added will let you erase everything, but that’s not what you want to do unless you have a complete backup. This is a wonderful XY question. You asked how to get past problem Y and we really needed to clarify X - what you actually are trying to do and what your constraints are.
    – bmike
    Commented Apr 30, 2020 at 10:30

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