How to Set Up a USB Key to Install CentOS

Starting with CentOS 6.5 and 7.0 the preferred way to create a USB stick to install from is by using dd (example below).

<!> Some of the Alternatives at the bottom are preferred by other users, and are demonstrated to work with CentOS-6.

The procedures below have been tested by various users, but may not cover all eventualities. There is a formal CentOS project recommended approach and that is to use dd.


Many recent systems, particularly netbooks and small notebooks, may not have a CD or DVD drive and a network install may be difficult, impractical, or impossible, depending on network connectivity and installer support for the available network hardware. This procedure allows a CentOS install without network connectivity and with no media other than a bootable USB device and the target system disk.

CentOS release 6 (newer than 6.5) and CentOS 7

Starting with CentOS 6.5, one can install from USB keys by simply transferring the desired ISO using dd.

For example, assuming your USB stick is seen as /dev/sdb (please double check what yours is, do not blindly assume /dev/sdb as you may overwrite something irretrievably):

dd if=CentOS-6.5-x86_64-bin-DVD1.iso of=/dev/sdb

You must write to the entire device and not a partition on it (so, /dev/sdb not /dev/sdb1)

When asked for the media to install from, select "hard disk" and then the device corresponding to the USB key.

Make sure you select as destination the device corresponding to the USB key (/dev/sdb in the above example) and not a partition (such as /dev/sdb1)

Exactly the same method works for CentOS 7. Moreover, the CentOS 7 installer image has a special partitioning which, as of July 2014, most Windows tools do NOT transfer correctly leading to undefined behaviour when booting from the USB key. Applications known (so far) to NOT work are unetbootin, multibootusb and "universal usb installler" - do NOT use these. Confirmed as functioning correctly are Rufus (may depend on options selected, there have been reports of failure with rufus too), Fedora LiveUSB Creator, Win32 Disk Imager, Rawrite32 and dd for Windows. If using a version of Windows newer than 7, make sure you unmount the USB drive first (formatting it prior to launching the disk copier is one way to accomplish that), otherwise Windows might refuse to write on the stick, bailing out with the "can't write to drive" error message.

If using dd for Windows, run dd --list and look carefully at the list of NT Block Device Objects and use the one that looks like \\?\Device\Harddisk1\Partition0 where the description is something like Removable media other than floppy. Block size = 512. Be very careful about which output device you pick or you may overwrite something you did not intend to! On my machine I ran dd if=CentOS-7.0-1406-DVD.iso of=\\?\Device\Harddisk1\Partition0 - your device names and command may vary accordingly!

Previous versions of CentOS 6

Beware that only the very latest CentOS releases are supported. We strongly advise you to not install anything but the latest minor release. Therefore the following methods should no longer be attempted unless you have a very very good reason to install an old and unsupported release.

An end user recommends the following approach for CentOS-6, using livecd-iso-to-disk from livecd-tools with DVD1. This has been tested with livecd-tools-13.4-1.el6 from EPEL. Thanks to forum user AndrewSerk for the recommendation in a forum post. See also the notation of a need for installation of qemu in this mailing list post.

Older Method

<!> This method has been reported as still viable for CentOS 6.4

Building a distribution on a USB key as of CentOS 6.2 - thanks to Mark Roth for the CentOS-6 procedure, and to Yves Bellefeuille on the CentOS-Docs list for several additions and corrections.

Recommended: An 8GB or larger USB key.

  1. Partition the USB key into two partitions: the first, FAT32, and about 250M; the second, the rest of the drive, and ext3. (An ext3 partition is required because the ISO file, that will later be copied to it, is larger than the maximum file size for a FAT32 or VFAT partition.)
  2. Mount DVD1, if it isn't automounted (on /media, or /mnt). The following assumes /mnt/USB and /mnt/DVD.
  3. Mount the USB key on /mnt/USB/.

  4. Copy the directory and contents of /mnt/DVD/isolinux to /mnt/USB/.

  5. Rename /mnt/USB/isolinux to /mnt/USB/syslinux

  6. Rename /mnt/USB/syslinux/isolinux.cfg to /mnt/USB/syslinux/syslinux.cfg

  7. Copy the directory and contents of /mnt/DVD/images to /mnt/USB/. (A bug has been filed [ToDo - add link] because as of 11 Jan 2012, the install requires the .iso (see below), but linux rescue requires the contents of images to load and run.)

  8. Assuming that the USB key is /dev/sdb
    syslinux /dev/sdb1 
  9. Unmount the first USB partition, and mount the second
    umount /mnt/USB
    mount /dev/sdb2 /mnt/USB 
  10. Copy the .iso file to /mnt/USB. Do not use the LiveCD or LiveDVD. Use DVD1, the minimal CD ISO, or a combined DVD1/DVD2 created following CD to DVD Media. Do not copy the contents - the install now wants the .iso file itself, which it mounts during installation, and follow the upstream Installation_Guide to create an images directory on the same partition. Optionally, add DVD2 if you used DVD1, and need it for the install.

  11. Optional: On /mnt/USB, create a grub.conf. This may be required if your system wants the USB key as the first drive, so that you can later copy it to the hard disk. See HowTos/GrubInstallation section 2 for guidance.

  12. Umount the USB key, and it's ready to use.
  13. Boot from the USB media by setting it as the first BIOS boot device, or on some BIOSs by hitting a key such as F12 to select a boot device after POST.
  14. During the installation process, the user is asked "What type of media contains the installation image?" Select the first partition on the USB key, which should appear in the menu under "Hard drive", then /dev/sda1 or whatever device corresponds to the first USB partition.

  15. After partitioning, the user is asked whether to install the Grub boot loader and where to install it. After booting from the USB key, the BIOS may think that the USB key is the first drive. If the USB is seen as the first device, then to install the Grub boot loader on the hard drive MBR, which is the usual case, the user must change the order of the hard drives using the Grub advanced installation options.
  16. After the Grub installation options, the following error message may appear: "Missing ISO 9660 image: The installer has tried to mount image #1, but cannot find it on the hard drive". The installation program is looking for the ISO file on the first partition of the USB key, but it's on the second partition. Go to a terminal with a shell with Ctrl-Alt-F2, unmount the first partition of the USB

    umount /mnt/isodir
    mount the second partition on the USB device. This will be the same device used in #14 above - for example
    mount -t ext3 /dev/sda2 /mnt/isodir
    Return to the installation program (Ctrl-Alt-F6) and choose "Retry".
  17. Finish the installation and reboot without the USB device connected. If there are problems booting then it is likely the boot record was written to the wrong device. See How to re-install bootstrap code (GRUB), and if having difficulties determining the correct device, Troubleshooting GRUB Issues.


<!> These alternative methods are listed as a historical convenience. They mostly apply only to CentOS 6 and should not be used for CentOS 7. The best way to copy a CentOS ISO image to a USB stick is using dd.

UNetbootin is probably the most popular "outside" method, has both Windows and Linux executables, and allows you to create bootable Live USB drives for a variety of Linux distributions. This utility is no longer recommended for use as it fails to work with newer CentOS iso images. There was an anaconda bug that could render the media unusable - now closed for Fedora 13 and presumably thus RHEL/CentOS-6. It has been reported on the fora to work with at least the netinstall ISO for CentOS-6. When using the netinstall ISO the full installation media ISO can be copied to the root directory of the USB media and a "Hard Drive" install performed.

Much better is livecd-iso-to-disk which can be obtained from git:

Just download it, chmod +x and run:

./ /path/to/iso /dev/partition_of_your_usb_stick

You could also add an option like:

--overlay-size-mb 1024

to add some persistent space to the key.

The resulting key could directly be used for installations, without the need of placing the iso manually on the key.

Using Windows

First of all, take into account that the instructions for modern CentOS versions ( CentOS 6 > 6.5, CentOS 7 ) are at the top of this page.

Starting with CentOS 6.5, one can create a bootable USB key simply by installing the ISO file on the key using a program such as Win32 Disk Imager. This will delete all information already on the key.

HowTos/InstallFromUSBkey (last edited 2017-05-19 15:24:18 by TrevorHemsley)