Bootable drive - disk is locked error 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. 


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


https://support.apple.com/en-us/HT204904


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).