After running the create media command, I am unable to eject the volume because it says in use. Doing some digging finds that ShareSupport.dmg is still in use. Any attempt to force eject or kill process corrupts the install media.
diskimage 848 root 3r REG 1,13 12440916552 770 /Volumes/Install macOS Big Sur/Install macOS Big Sur.app/Contents/SharedSupport/SharedSupport.dmg
A bit more digging found that SharedSupport.dmg was mounted multiple times, but ultimately were dismountable via Disk Utility. Strange that the media creation tool would act this way, but at least this looks like a work around.
Have created Bootable Installers of Big Sur 11.01, 11.3.1, 11.5 and latest 11.5.2 and on M1 Macs. You are correct in un-mounting the the Installer Drive in the normal method does not seem t work.
Have always had to use the oldest trick - shutdown computer - remove the USB Installer Disk and restart the computer.
Old is Gold, in this situation.
Created install media will not mount - In use.
