general storage

ISO Image (ISO)

iso disk-image install-media bootable-usb linux
Plain English

An ISO image is one file that contains a whole installer disc. When you download Ubuntu or Windows, you get an ISO. You can attach it to a virtual machine as if it were a DVD, or write it to a USB stick to install on a real computer. Because it is one file, it is easy to share and easy to verify with a checksum before you trust it.

Technical Definition

An ISO image is a sector-by-sector archive of an optical disc, named after the ISO 9660 filesystem standard. Modern distribution ISOs are hybrid images: they contain both an ISO 9660 filesystem and a partition table with an EFI System Partition, so the same file boots from a DVD, a USB stick, or a VM’s virtual optical drive, in both UEFI and legacy BIOS modes.

Typical Linux installer ISO contents:

  • A bootloader (GRUB and shim for UEFI)
  • A kernel and initramfs
  • A compressed root filesystem (squashfs) for the live session
  • The installer and package pool

Using an ISO:

  • VM: attach it to the virtual optical drive (VirtualBox: Settings, Storage, Optical Drive)
  • USB: write it raw to the whole device with balenaEtcher, Rufus (in DD mode), or dd. Copying the file onto a formatted USB stick does not make it bootable.
  • Inspect: mount it read-only with mount -o loop

Verify before use: compare its SHA-256 against the publisher’s SHA256SUMS, and check that file’s GPG signature.

Source: Ubuntu: How to verify your Ubuntu download

Verify and write an ISO 

# Verify the checksum against Ubuntu's published list
$ sha256sum -c SHA256SUMS --ignore-missing
ubuntu-26.04-desktop-amd64.iso: OK

# Find the USB stick. Check SIZE carefully; dd overwrites without asking
$ lsblk -d -o NAME,SIZE,MODEL
NAME   SIZE MODEL
sda    500G Samsung SSD
sdb   14.9G USB Flash Disk

# Write it to the whole device, not a partition (sdb, not sdb1)
$ sudo dd if=ubuntu-26.04-desktop-amd64.iso of=/dev/sdb bs=4M status=progress oflag=sync

# Look inside without writing anything
$ sudo mount -o loop,ro ubuntu-26.04-desktop-amd64.iso /mnt && ls /mnt
In the Wild

ISOs are how almost every OS install begins: Ubuntu, Windows, Proxmox VE, TrueNAS, pfSense. In a homelab you keep a folder or a Proxmox storage pool full of them. The two mistakes that cost the most time are skipping the checksum, which lets a corrupted download fail halfway through an install, and pointing dd at the wrong disk, which erases it. Tools like Ventoy let one USB stick hold many ISOs and boot whichever you pick.

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