Here’s a guide to installing a bootable CD-ROM ISO image as a bootable partition on a (USB) drive, while using other partitions as normal filesystems. The most complicated part is partitioning and formatting the drive, which is well documented elsewhere, but I didn’t see anyone else explaining exactly how to do this trick.
Use Cases and Alternatives
If you just want an OS on a drive you can carry around, try just installing the OS normally on a portable drive. Most FOSS OS installers are flexible enough to allow that. System upgrades will work as normal.
If you just want to use an existing live CD ISO, but with the ability to save data, you can extract the files from the ISO onto a regular drive, and boot the drive with an appropriate bootloader. unetbootin is a classic tool for doing this automatically, and you can also do it manually. This works with common *nix live CDs, but isn’t guaranteed to work with arbitrary bootable CDs. You also may or may not be able to upgrade this system easily.
If you just want to customise a CD a bit, in principle you can extract the files, tweak them and then rebuild an ISO. This should work with practically any CD, and there’s a detailed example in the SystemRescueCd docs. I’ve tried this before, but in practice it gets tiring to rebuild an ISO every time a file needs to be changed. Also, merging changes with an upgraded version of the CD might be difficult.
I use a multi-partition bootable USB drive as a system recovery drive. It has a recent install CD image written to partition #1, and a bunch of my own config files and scripts, etc, on a normal ext filesystem on partition #2. The ISO can be updated by downloading a new one and writing it using the tools below, and I can update my files by mounting the second partition as a normal drive. I actually had to use this drive last week for the second time when my computer’s motherboard died and I had to reinstall my stuff on a new machine. It was a useful tool, and that’s what motivated me to write this guide.
Another possibility is installing multiple live CDs to one drive – say, for a demo. I’ve never needed to do this, but you’d want to install a chaining bootloader on one partition to provide a nice menu for booting the other partitions. That’s not covered by this guide.
Writing a live CD ISO to a disk will practically destroy all data that was on it. Of course, that’s fine if you’re writing to a blank drive you just bought from a shop, but if you mess up and write to your main hard drive instead, then complaining to a blogger in Australia won’t help you get your data back. I can’t take responsibility for anything you might break, so if you’re not comfortable, work on a new virtual machine or something else you’re willing to lose.
If you’ve never done any partitioning or formatting of disks before, I recommend reading one of the many guides out there.
The specific commands I give are for a *nix system, but if you know what you’re doing, I guess you could make it work on Windows (maybe with help from Cygwin).
Many of these commands will require root or administrator rights.
1. Prepare the ISO
This part doesn’t need root privileges.
isohybrid is a
tool from the Syslinux project that makes ISOs bootable from regular (non-optical) disks. It can also make ISOs
bootable from a partitiion if you use the
line flag. Your OS’s package manager probably has it under
Here’s an example using a copy of SystemRescueCd:
The file is modified in place.
If you came here just wanting to know how to make a bootable USB drive, and don’t care about creating extra
partitions, then you can make things much simpler. Just run
isohybrid on a bootable ISO without the
--partok, and write it directly to the drive just like in the MBR
installation step below.
2. Partition the drive
If the warning above didn’t make it clear: make sure you know which drive you want to write to. Do not just
copy my examples. On a *nix system, the drive will probably have a name like
/dev/sdb or something, but check your OS documentation. You want the
disk, not any of its partitions (e.g.,
/dev/sdc2). On a modern *nix system, the device name will
/dev if you remove the drive, and reappear if
you plug it in again.
Another way to check is to try reading data from the drive (as root):
Many USB drives have an LED that’ll flash like mad when you do this, and stop when you kill the read with
Ctrl+C. It’s a good idea to run
mount without arguments and/or read
/etc/fstab to familiarise yourself with what drives are used as what. If
anything from the drive you’re planning to use is mounted, it’s saner to unmount it before messing with the partition
There are many tools for partitioning drives; some are friendlier than others. This guide uses old-fashioned MBR
partitioning, not any new-fangled GPT partitioning. It’s well-supported and good enough for a small USB drive. We want
one bootable partition that’s big enough for the ISO, and any other partitions as needed. Here’s an example session
fdisk on a new 8GB drive, creating a 1GB partition for
the live CD image, and reserving the rest for an ext4 filesystem:
3. Format the writable partitions
You can use any filesystem you fancy here (although some systems might complain if the partition type doesn’t
match). A basic FAT32 filesystem has broad support across operating systems and devices, but doesn’t have *nix features
like owners and permissions and links. ext4 is the most popular filesystem on GNU/Linux systems today, and it’s
suitable for my rescue USB drive. If you want, you can tinker with the various options in
man mkfs.ext4 and do things like, say, disabling journalling, but
realistically the out-of-the-box settings work fine for this job. My first partition is the boot partition, so
partition #2 is for files:
4. Write the ISO
After the ISO has been converted with
isohybrid, it just
needs to be written bit-for-bit to the boot partition (
/dev/sdz1 in my example).
dd is the command for that:
bs (“blocksize”) parameter is not critical.
5. Write a Master Boot Record (MBR)
The MBR is what makes the drive bootable. When you boot from the drive, it searches for a bootable partition and
hands over the boot process to it. The Syslinux project provides MBRs, which are tiny files (less than 512 bytes) of
mostly machine code. Debian also has a convenient package called
mbr that provides a simple command
install-mbr. The MBR needs to be installed to the start of the disk, not
to a partition. Here are some examples of installing to
/dev/sdz (only one command is necessary):
If all went well, the drive should now be bootable. You can mount the second partition of the drive as normal (e.g.,
mount /dev/sdz2 /mnt).
If you want to upgrade the CD image, just download the new ISO, convert it with
isohybrid, and write it to the boot partition. There’s no need to
reinstall the MBR, or redo any of the other steps.