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My goal: try to install Ubuntu on a separate partition of my laptop's HDD which would only be bootable with an inserted USB drive. This drive would contain GRUB and additional ext4 storage, without it the laptop will boot into w8.1 regularly.

My problem: constant installation failure. I've maxed the available space to 20gb on HDD, 16gb on USB. I reckon it could be that my HDD is bitlocker encrypted but I have yet to decrypt it and try again. I am booting with UEFI hybrid with CSM.

I'd like to know whether there's a problem with my concept or if there are some crucial technical details I'm missing.

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In linux and open source world, almost everything is possible 😁

bitlocker is supposed to encrypt a partition, not the whole HDD; so this should not be a problem.

So, assuming you are booting using UEFI (please specify that in your question by editing it), you can create an ESP on your USB drive alongside the ext4 partition. This means you need to reformat it using a GPT header, not MBR.

Just use gparted from the Live USB to format your USB drive:

  • Choose to create a new partition table with GPT
  • Create a vfat partition and add the boot flag and esp on it (this turn it into an ESP)
  • Create the ext4 partition

Then, during the installation, you point GRuB to be instaled on your USB ESP. I have not tested this.

When you boot your laptop, choose to boot from the USB, and this should boot straight away into ubuntu.

  • While grub in BIOS mode will install to drive you specify, in UEFI mode it only installs to first ESP, usually your Windows ESP. Backup ESP before install as grub will probably overwrite /EFI/Boot/bootx64.efi which is a fallback boot. It is a copy of Windows, but grub will make it a copy of shimx64.efi. After install you can then copy boot files to ESP on flash drive(not installer) and remove /EFI/ubuntu folder from ESP and restore /EFI/Boot file(s). You will only be able to boot from UEFI boot menu. And UEFI only boots flash drive from /EFI/Boot/bootx64.efi, but grub's shim needs /EFI/ubuntu. – oldfred Apr 3 at 14:22
  • Just tested it - It works!! – turboburger Apr 3 at 14:52
  • @turboburger it works as I said or you needed to use the trick of oldfred ? Please mark it as a answer by ticking the check mark. Thanks – solsTiCe Apr 3 at 14:54
  • @solsTiCe Apologies, very new to the platform. I've followed your instructions but was met with the same troubles mentioned in this question I've added a customized boot path which references oldfred solution, it points to the \EFI\ubuntu\shimx64.efi file present on the USB drive, his solution might also work but I'm settling for what I have now. I set boot priority to custom boot > USB > OS and it all works without a hassle! With the drive unplugged w8.1 boots and with it I go straight to Ubuntu! Many thanks! – turboburger Apr 3 at 15:03

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