UnetBootin and Mac OS X: fix “Invalid system disk” and similar errors

If you ever tried to create bootable USB stick with UnetBootin, I guess you faced a problem that some USB sticks fail to boot. Here’s the manual how to fix it.

Partition and format

First, make sure your USB stick has MBR partition scheme and FAT file system:

  1. Open Disk Utility
  2. Select the USB device on the left side of the window
  3. Select Partition tab
  4. Select 1 partition layout
  5. Select MS-DOS (FAT) format type
  6. Apply

Write MBR boot code

Most likely your USB stick drive doesn’t have actual MBR boot record.

  1. Open Terminal.
  2. To find the identifier of your USB device, type: diskutil list. It looks like /dev/diskX. Most likely /dev/disk0 is your system disk and /dev/disk1 is your USB device, but it may vary depending on the system. Remember or note it.
  3. Manually unmount drive: diskutil unmountDisk /dev/diskX
  4. Make partition active:
    sudo fdisk -e /dev/diskX
    f 1
  5. Unmount it again: diskutil unmountDisk /dev/diskX
  6. Get mbr.bin file. You can download it here or extract it from SysLinux. It should be exactly 440 bytes long.
  7. Copy mbr.bin into beginning: sudo dd conv=notrunc bs=440 count=1 if=~/Downloads/mbr.bin of=/dev/diskX. You may want to alter ~/Downloads/mbr.bin path if you moved mbr.bin file.


Use UnetBootin as usually, eject USB device and try to boot. If you don’t repartition this USB drive, next time you write it with UnetBootin it should boot fine without doing all these steps again.

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