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I want to install Ubuntu on a USB and run it from there while still being able to run my MacBook Pro (Retina, 13", Early 2015) normally when the USB is not inserted. In my previous attempt, my MacBook Pro started with grub by default (I think) and complained that the USB was gone so I have force shut down and press the option key whenever I use macOS (10.13.2). What should I do so I can run macOS by default and then just press the option key to use the USB running Ubuntu? I think this has something to do with grub so how do I install grub on the USB and not on my internal drive? Thanks!

Gabriel Christian Alava
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This is a duplicate. Take a look at Install Ubuntu to usb drive and boot it to Uefi and Bios system. The gist is bug 1173457, installer puts grub in the wrong place, sda. The fix is to copy the sda EFI partition's files to the USB's EFI, then copy the UEF's /EFI/ubuntu/grubx64.efi and /EFI/ubuntu/shimx64.efi to /EFI/Boot and rename /EFI/Boot/grubx64.efi to /EFI/Boot/bootx64.efi. Then use efibootmgr to reorder the boot entries on the hard disk to put USB first, Windows second, (or Windows first, and use the EFI menu (some function key at power up to select boot device).

ubfan1
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  • You do have to manually partition external drive in advance, or it will not have an ESP (FAT32 with boot flag). http://askubuntu.com/questions/743095/how-to-prepare-a-disk-on-an-efi-based-pc-for-ubuntu & https://askubuntu.com/questions/343268/how-to-use-manual-partitioning-during-installation – oldfred Dec 19 '17 at 17:12