How Does Ventoy Boot ISO Files Without Reformatting?

Ventoy eliminates the need to repeatedly reformat USB drives by separating the boot environment from the storage space and loading ISO files directly from a standard filesystem. Instead of extracting or overwriting the drive's contents each time an operating system is changed, Ventoy installs a persistent bootloader once, allowing users to add, update, or remove ISO images using standard file drag-and-drop actions.

The Limitations of Traditional Bootable USB Creation

Conventional bootable USB utilities, such as Rufus or the dd command, write disk image contents directly across the raw partition structure of a flash drive. This process extracts the image's filesystem or clones the raw blocks onto the drive, which overwrites any partition tables and destroys preexisting data. Consequently, switching to a different Linux distribution or Windows installer requires wiping the entire storage device, while simultaneously locking the remaining capacity out of everyday storage use.

The Dual-Partition Architecture

Ventoy resolves this limitation during its initial installation by establishing a dual-partition layout across the USB drive:

• The Bootloader Partition: A small, protected partition formatted as FAT/EFI. This partition houses core boot components, including GRUB-based loaders, UEFI shims, and hardware initialization modules. It remains untouched during regular computer usage.
• The Data Partition: A primary partition taking up the rest of the available storage, formatted in exFAT, NTFS, ext4, or FAT32. Operating systems see this partition as a regular storage drive. ISO, WIM, VHD, or IMG files can be pasted directly into this partition alongside regular personal documents.

In-Memory Virtualization and Dynamic Hooking

When a computer boots into the Ventoy drive, Ventoy executes from its hidden boot partition and scans the adjacent data partition for supported image files. It populates a boot menu automatically without requiring configuration file edits.

The core technical breakthrough occurs after an image is selected:

1. Virtual Drive Emulation: Ventoy creates a virtual disk in memory that maps directly to the continuous block addresses of the selected ISO on the physical USB drive.
2. BIOS and UEFI Interception: Through runtime hooks (such as INT 13h routines for legacy BIOS or EFI protocol wrappers for modern UEFI systems), Ventoy intercepts disk access requests. When the booting kernel queries storage hardware for installation media, Ventoy presents the virtual disk rather than forcing the operating system to search for physical optical media.
3. Initramfs Patching: For Linux distributions, Ventoy injects small runtime patches into the boot sequence, ensuring the operating system mounts the internal container image from the USB filesystem seamlessly.

By running the emulation entirely in memory during the pre-boot phase, Ventoy preserves the underlying files as static data. Users can manage multiple bootable operating systems simultaneously simply by manipulating ordinary files within file explorer.