Sharing Files Between Ubuntu and Windows on Bootable USB

Sharing Files Between Ubuntu and Windows. Want to share files between Linux and Windows using a bootable USB drive? By default, Ubuntu’s casper script mounts the USB drive in read only mode for non root users. This prevents saving files to the USB while running Ubuntu live from a flash drive, whereas we want to be able to access USB from Windows and Linux.

However, with a simple tweak to the casper script, you can enable full read and write access; even for the default "live" user. This allows you to easily copy, move, and share files between Ubuntu and Windows using the same USB stick.

Easily Sharing files Between Ubuntu and Windows

In this simple tutorial, you’ll learn how to modify the initrd.lz file to allow saving files from Ubuntu back to the USB drive, and how to access those files again from Windows or Linux.

This method of sharing files between Ubuntu and Windows should work on Ubuntu 11.10, 11.04, 10.10, and 10.04, as well as compatible distributions like Kubuntu, Linux Mint, and other Debian based remixes that use casper.

How Sharing Files Works

Ubuntu’s boot process uses a compressed initramfs (initrd.lz) that includes a casper script for mounting and managing devices. By editing this script, you can change the USB mount parameters, enabling full access for the default user. This means you can:

• Save and access files on /cdrom (USB root)
• Share files between sessions
• Add files from Windows and retrieve them later in Linux

Prerequisites for Sharing Files Between Linux and Windows

Before starting, you’ll need:

• A working bootable USB with Ubuntu
• Basic familiarity with terminal commands
• Time to reboot and test changes

How to Enable Read/Write USB Access in Ubuntu Live

1. Boot into Ubuntu from your USB Flash Drive
2. Open a terminal and type:
   sudo su
3. Create a working directory:
   mkdir /projectaccess
cd /projectaccess
4. Extract the initrd.lz file:
   lzma -dc -S .lz /cdrom/casper/initrd.lz | cpio -id
5. Edit the casper script:
   gedit scripts/casper
6. In gedit, find:
   home_snapshot_label="home-sn"
   Add below it:
   mountmode="ro,noatime"
7. Then find (based on your version):
   • For Ubuntu 10.04 or 10.10:
     IGNORE_UUID="Yes" ;;
   • For Ubuntu 11.04 or 11.10:
     UUID="" ;;

   Add below it:

   basemountmode=*)
mountmode="${x#basemountmode=}";;

8. Now find:
   if is_supported_fs ${fstype}; then
   Change the mount line directly below it to:
   mount -t ${fstype} -o ${mountmode} "${devname}" $mountpoint || continue
9. Save the file and return to the terminal
10. Rebuild the initrd.lz file:
    find . | cpio -o -H newc | gzip -9 > initrd.lz
11. Copy the new file back:
    cp initrd.lz /cdrom/casper
12. Edit the boot configuration:
    gedit /cdrom/syslinux/txt.cfg
13. Locate the first append line and replace:
    initrd=/casper/initrd.lz
    With:
    initrd=/casper/initrd.lz basemountmode=ro,noatime,uid=999,gid=999
14. Note: On Ubuntu 10.04, the file might be named text.cfg
15. Save and exit
16. Reboot and boot again from your USB

What You Can Do Now

• Save documents, media, or configs from Linux to your USB
• Access files added in Windows from Linux
• Create persistent like storage without needing a separate casper-rw partition

Troubleshooting Tips

• Not saving to /cdrom? Double check your mountmode and casper edits.
• Permission issues? Ensure you're using uid=999,gid=999 for the default user.
• File system not writable? Make sure the USB is formatted FAT32 or a supported writable file system.

Frequently Asked Questions

Can I use a bootable USB to transfer files between Ubuntu and Windows?

Yes, by modifying the casper script, you can enable read/write access and use your USB drive to allow for sharing files between both operating systems.

Why can't I write files to the Ubuntu USB drive?

By default, Ubuntu Live mounts the USB drive in read only mode for non root users. This guide shows how to change that behavior and enable write support.

Is this method better than persistence for sharing files?

Yes, especially for quick file transfers. It allows simple sharing between Windows and Linux without the need for a separate casper-rw persistence partition.

Does this work with Linux Mint or other Ubuntu-based systems?

Yes. This method works with Linux Mint, Kubuntu, and most Debian-based distributions that use the casper boot system.

Final Thoughts

With this modified casper setup, your Ubuntu Live USB now acts as both a bootable OS and a file sharing medium. This gives you true flexibility to carry files across systems, tweak configs on the fly, or simply back up your data. Kudos to Julian Chennales for submitting the original technique of sharing files to Pendrive Linux, which has since been kept updated by Lance.

Bonus Tips: Want persistence too? Try YUMI exFAT to boot multiple ISOs and retain settings between sessions. Learn more about the casper boot system options from the official Ubuntu documentation.