Booting other OSs on an external disk

KenWong wrote:

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I am wondering whether I can attach that hard drive to my MacBook Pro 2018 and boot it from any of the OSs on the external hard drive. Is there a way to make it?

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If it is not a GUID partition scheme it will not be bootable. While Macs can read drives with an MBR partition scheme, they don't recognize MBR partitioning as being bootable.

And at that, external drives have to have a special "blessing" applied to them to make them bootable.

In addition, since none of the OSes will have any Mac drivers, they may or may not boot at all. Generally the drivers need to be initially installed when the OS is installed.

One other possibility, there are various boot utilities available that can boot different filesystems, i.e. like GRUB. Some can be resident on a USB stick but some need to be installed on the internal drive. In the latter make sure, you have a good back up of your Mac system in case some thing goes wrong.

An additional alternative would be to install VMware Fusion or Parallels Desktop Mac. You could possibly attach the OS partition as a VM source. Windows may have the most success at that and may actually be able to import it into a VM format.

See this previous discussion:

https://discussions.apple.com/thread/253389720

Install operating system from external USB stick

from that info I do not think you can directly boot from the old external drive but you seem to be able to install Linux onn external drive and boot from it.

Outside of all the excellent advise from previous posting is another possible issue About Startup Security Utility on a Mac with the Apple T2 Security Chip only if this computer is equipped with the T 2 / Touch ID Chip