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How to create bootable USB stick

Bootable USB stick can be useful for things like upgrading motherboard BIOS or flashing CD/DVD/HDD firmware. 

Here is how to create one using Mac OS X

Connect the FlashDrive. The FlashDrive will be erased so please make a backup before continuing.
Download the desired .img file (FreeDOS for example - fdboot.img mirror)
Open a terminal window - Applications > Utilities > Terminal
To get the list of devices enter the following:
diskutil list

You will get something similar to this:
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.1 GB   disk0
   1:                        EFI                         209.7 MB   disk0s1
   2:                  Apple_HFS mini500                 499.8 GB   disk0s2
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *1.0 GB     disk1
   1:               Windows_NTFS 1GB                     1.0 GB     disk1s1

As you can see my FlashDrive is /dev/disk1
Yours may be different so please make a note of it because you will need it later.

Unmount your FlashDrive. Make sure to use your /dev/disk#
diskutil unmountDisk /dev/disk#

Copy the image to the FlashDrive
dd if=/path/to/downloaded.img of=/dev/disk# bs=1m
dd if=~/Downloads/fdboot.img of=/dev/disk1 bs=1m

You will get something similar to this:
1+1 records in
1+1 records out
1474560 bytes transferred in 0.717656 secs (2054689 bytes/sec)

Eject FlashDrive
diskutil eject /dev/disk#

The FlashDrive can be disconnected now. It is ready to use.
If you decided to use fdboot.img provided above your FlashDrive has full read/write permissions. This means you can copy/delete any files from it and be able to re-use it. You can for example copy new BIOS for your motherboard or CD/DVD/HDD firmware updates.

Note: Some motherboards have problems booting up from certain USB sticks. In case of problems you may try USB stick from a different vendor.