Boot a Linux ISO from Your Hard Drive: GRUB Guide

You can boot a Linux ISO from your hard drive without first writing it to a USB stick—provided your existing GRUB installation can read the file and the distribution supports this boot method. It is useful for testing a live desktop or keeping a recovery image close at hand.

The important detail is that GRUB opening an ISO is only the first half of the job. After the kernel starts, the live system must find that same ISO on the physical disk. This guide explains both stages, using an Ubuntu-style live image as the example.

Updated: 31 October 2026. Scope: an existing Linux installation using GRUB 2. Commands below are an adaptable configuration example, not a hardware-tested recipe for every ISO or firmware combination.

Which Linux ISO boot method should you choose?

Method	Best use	Main consideration
GRUB loopback boot	Starting a compatible live ISO stored on an internal disk	Requires an existing GRUB installation and image-specific boot options
Virtual machine	Exploring a distribution while your current OS stays running	Does not test native hardware compatibility in the same way
Ventoy USB	Carrying several installer or rescue images	Installing Ventoy normally formats the selected USB device
Distribution’s standard bootable USB	A straightforward installation or recovery path	Needs a USB device and the vendor’s current instructions

For occasional testing, a VM is usually the easiest starting point. Choose GRUB loopback when you specifically need a live boot from local storage. Ventoy is a convenient alternative for a multiboot USB; follow its official installation instructions and check the selected disk carefully.

Before you change GRUB

• Keep a recovery route: back up important files and have working rescue media available. Do this on a personal or lab machine before attempting it on a critical workstation.
• Match the architecture: an amd64 image targets x86-64 Intel/AMD systems; an arm64 image requires a compatible ARM platform.
• Check the boot chain: a Windows-only machine without GRUB needs a different preparation process. This article does not install or replace your bootloader.
• Use readable storage: GRUB and the live initramfs both need access to the ISO. Encrypted volumes, RAID, unusual filesystems and separate mounts add complexity.
• Keep the source partition intact: do not format or delete the partition holding the ISO while an installer is using it.

UEFI and Secure Boot are separate from ISO compatibility. Signed boot components, firmware trust and the installed GRUB build all matter. If you encounter a verification error, use the distribution’s supported signed boot path; disabling Secure Boot should not be the default troubleshooting step.

How GRUB loopback boot actually works

The GNU GRUB manual makes this distinction clear: reading an image does not automatically make the operating system bootable from it. A distribution’s initramfs must understand how to locate its live root filesystem. That is why an Ubuntu casper entry should not be copied unchanged for Fedora, Debian Live, Windows or an arbitrary rescue ISO.

1. Download and verify a current Linux ISO

Download from the distribution’s official website. At the time of this update, Ubuntu lists 26.04.1 LTS and 26.10 on its desktop download page. Choose the release and support model appropriate to your needs, then use the exact filename you downloaded throughout the steps below.

For Ubuntu, obtain the matching SHA256SUMS and SHA256SUMS.gpg. Follow Canonical’s image verification procedure to validate the signing key and checksum-file signature first. Then check the ISO from the directory containing those files:

sha256sum -c --ignore-missing SHA256SUMS

Your downloaded ISO should report OK. A checksum verifies integrity against the checksum file; authenticating that file is what connects the result to a trusted publisher.

2. Put the ISO on accessible storage and inspect it

For a simple Ubuntu/Debian-style lab setup, use a folder such as /iso on an unencrypted filesystem accessible to the bootloader. The following filename is an example, not a download command:

sudo mkdir -p /iso
sudo cp ~/Downloads/ubuntu-26.04.1-desktop-amd64.iso /iso/ubuntu-live.iso
findmnt -T /iso/ubuntu-live.iso
lsblk -f

Record the filesystem containing the file. Linux mount paths and GRUB paths are not always identical: if a partition mounted at /data contains /data/iso/ubuntu-live.iso, its partition-relative path is /iso/ubuntu-live.iso. Keeping the example on a simple root filesystem avoids that extra translation.

Mount the image read-only and inspect its own boot configuration:

sudo mkdir -p /mnt/linux-iso
sudo mount -o loop,ro /iso/ubuntu-live.iso /mnt/linux-iso
ls /mnt/linux-iso/casper/
ls /mnt/linux-iso/boot/grub/
less /mnt/linux-iso/boot/grub/grub.cfg

Also inspect boot/grub/loopback.cfg if present. Confirm the kernel and initramfs filenames and any live-filesystem arguments. Ubuntu’s published 26.04.1 desktop image file list provides another way to check image contents. Do not assume old names such as vmlinuz.efi or initrd.gz still apply.

3. Add an Ubuntu-style GRUB ISO boot entry

On Ubuntu/Debian installations using /etc/grub.d/40_custom, back up the file before editing it:

sudo cp -a /etc/grub.d/40_custom /etc/grub.d/40_custom.before-linux-iso
sudoedit /etc/grub.d/40_custom

Preserve the existing file header and append a menu entry. This example assumes the ISO contains /casper/vmlinuz and /casper/initrd:

menuentry "Ubuntu live ISO — local disk" {
    set isofile="/iso/ubuntu-live.iso"
    search --no-floppy --file --set=root $isofile
    loopback loop $isofile
    linux (loop)/casper/vmlinuz boot=casper iso-scan/filename=$isofile
    initrd (loop)/casper/initrd
}

Adapt this before rebooting. Copy the appropriate additional kernel options from your ISO’s live/loopback configuration. Some desktop images use layered live filesystems and need release-specific options. The Ubuntu community ISO boot guide contains examples but also historical release details; the image you actually downloaded is the final check.

• search --file locates a filesystem containing the named ISO instead of assuming a disk number. Keep the path unique if several disks contain copies.
• loopback exposes the ISO’s contents to GRUB. The command is loopback, not lookback.
• iso-scan/filename tells an Ubuntu-style live system where to look for the ISO after the kernel starts.

The GRUB search reference also documents searching by filesystem UUID, useful when file-based selection would be ambiguous.

Regenerate the configuration on Ubuntu/Debian:

sudo update-grub
sudo grub-script-check /boot/grub/grub.cfg

A successful syntax check does not prove the ISO will boot. Confirm the generated menu entry and file paths before rebooting. Other distributions can use different configuration locations and generation commands—do not substitute a Fedora/RHEL bootloader command without checking that system’s documentation.

When finished inspecting the mounted ISO:

sudo umount /mnt/linux-iso

Reboot, keep your normal operating-system entry available, and select the new live ISO entry. Start with the live session before launching any installer.

4. Troubleshooting Linux ISO boot errors

Symptom	What to check
ISO file not found	Exact spelling, filesystem-relative path, duplicate copies, and whether GRUB can read the storage
Kernel or initrd not found	Actual filenames inside the image; do not reuse paths from an older release
Unable to find a live filesystem	The initramfs cannot rediscover the ISO, or required image-specific live-boot arguments are missing
Secure Boot verification failure	Signed kernel/bootloader compatibility and the distribution’s supported boot method
New menu entry does not appear	Whether the custom script is included and configuration was regenerated for the active GRUB installation

If the ISO entry fails but the normal OS entry still works, boot the normal OS, remove only the newly added entry from 40_custom, and run sudo update-grub again. Restore the backup only if no other changes have been made to that file since it was created. If the installed system no longer boots, use your rescue media and the distribution’s recovery instructions.

Linux ISO booting: frequently asked questions

Can I boot any ISO directly from GRUB?

No. GRUB’s ability to read an ISO is different from the operating system’s ability to start from it. Check distribution-specific live-boot support.

Does booting a live ISO install Linux?

No. Starting a live session and running an installer are separate actions. An installer can modify or erase disks depending on the choices you make.

Will changes survive a reboot?

Not automatically. Persistence requires explicit support and configuration for that particular live image. A normal ISO file remains a read-only image.

Can I use this with UEFI and Secure Boot?

Potentially, but compatibility depends on the trusted boot chain and the image. A universal GRUB snippet cannot guarantee Secure Boot compatibility.

Is GRUB loopback better than Ventoy?

They solve different problems. GRUB loopback is useful for a compatible image on an existing disk; Ventoy is convenient for maintaining several boot images on a USB drive.

Continue learning

• Ubuntu kernel panic: boot troubleshooting guide
• Ubuntu system restart required: check reboot status

For a reliable local ISO boot setup, focus on three checks: a verified download, a storage path both GRUB and the live system can access, and boot arguments taken from the correct image.