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My goal was to have a Linux Ubuntu on a USB (not live CD but a full installation) so that whenever i want, instead of booting Windows, i can just insert the USB and my computer will just boot this ubuntu with all the storage on it.

I installed ubuntu on a 64GB USB, following this guide

In short: Installing the ubuntu ISO on one small USB, boot from that -> Try Ubuntu.
From the Live CD - installing ubuntu on another USB (64GB) which includes formatting and partitioning the USB properly.

Everything went fine, except for when i was starting my computer without the USB, i reached a terminal that looked something like: 

                        GRUB version 2.x

#>

I figured that if i just type exit the computer will start Windows. But i wasn't very happy with this thing. So i followed this video which explains how to remove GRUB. So now when i turn on the computer i doesn't show that GRUB prompt, but on the other hand, when i start the computer WITH the usb in, it doesn't boot ubuntu, although my BIOS is configured to boot from the USB first.

My question is, why do i need to have GRUB installed on my PC so i can boot linux from the USB?

YoavKlein
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    Ubuntu's Ubiquity installer only installs UEFI grub to first drive.You need to have an ESP on external drive and reinstall grub, often easier with Boot-Repair. You can add ESP with gparted Posted work around to manually unmount & mount correct ESP during install #23 & #26 https://bugs.launchpad.net/ubuntu/+source/ubiquity/+bug/1396379 Others suggest disconnecting all other drives physically or logically in UEFI settings, so install drive is first drive. Or removing boot flag/esp flag from first drive, so only ESP is install drive. – oldfred Oct 06 '20 at 19:38
  • What do you mean by "first drive"? my hard drive? – YoavKlein Oct 06 '20 at 20:25
  • Whatever order UEFI/BIOS gives. Usually the internal drive that has Windows. But some systems make USB flash drive as first drive & grub installs to that, corrupting installer if you want to reuse installer. Best to see what drive is what during install process. – oldfred Oct 06 '20 at 21:09
  • It is much easier to flash a prebuilt Ubuntu 20.04 image file to USB than to build one from scratch: https://askubuntu.com/questions/1279465/is-it-possible-worthwhile-to-install-ubuntu-on-its-own-ssd-so-it-can-be-swapped/1279622#1279622 The instructions you used do not even mention UEFI. For step by step see: https://askubuntu.com/questions/1217832/how-to-create-a-full-install-of-ubuntu-20-04-to-usb-device-step-by-step – C.S.Cameron Oct 07 '20 at 04:44
  • I noticed that the MBR of the USB is empty. Can I install a first-stage GRUB in the MBR and set UEFI to boot from USB first? – YoavKlein Oct 12 '20 at 06:06

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