Making Installation USB Media on Mac OS X

This procedure involves using the dd command line tool to write the installation image to a USB flash drive. Note that some steps involve use of the sudo command, which is only available when logged in with an administrator account that requires a password.

All data on the USB flash drive will be deleted by this procedure.

Making USB Media on Mac OS X

Connect a USB flash drive to the system and identify the device path with the diskutil list command. The device path has the format of /dev/disknumber, where number is the number of the disk. The disks are numbered starting at zero (0). Disk 0 is likely to be the OS X recovery disk, and Disk 1 is likely to be your main OS X installation. In the following example, it is disk2:

$ diskutil list
/dev/disk0
#:                       TYPE NAME                    SIZE       IDENTIFIER
0:      GUID_partition_scheme                        *500.3 GB   disk0
1:                        EFI EFI                     209.7 MB   disk0s1
2:          Apple_CoreStorage                         400.0 GB   disk0s2
3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
4:          Apple_CoreStorage                         98.8 GB    disk0s4
5:                 Apple_Boot Recovery HD             650.0 MB   disk0s5
/dev/disk1
#:                       TYPE NAME                    SIZE       IDENTIFIER
0:                  Apple_HFS YosemiteHD             *399.6 GB   disk1
Logical Volume on disk0s1
8A142795-8036-48DF-9FC5-84506DFBB7B2
Unlocked Encrypted
/dev/disk2
#:                       TYPE NAME                    SIZE       IDENTIFIER
0:     FDisk_partition_scheme                        *8.0 GB     disk2
1:               Windows_NTFS SanDisk USB             8.0 GB     disk2s1

To identify your USB flash drive, compare the NAME, TYPE and SIZE columns to what you know about your flash drive. For example, the NAME should be the same as the title of the flash drive icon in the Finder. You can also compare these values to those in the flash drive’s information panel; right-click on the drive icon and select Get Info.

Use the diskutil unmountDisk command to unmount the flash drive’s filesystem volumes:
```
$ diskutil unmountDisk /dev/disknumber
Unmount of all volumes on disknumber was successful
```
When you do this, the icon for the flash drive disappears from your desktop. If it does not, you might have identified the wrong disk. If you attempt to unmount the system disk accidentally, you get a failed to unmount error.

Use the dd command as a parameter of the sudo command to write the ISO image to the flash drive:

$ pv /path/to/image.iso | sudo dd of=/dev/rdisknumber bs=1m

Mac OS X provides both a block (/dev/disk*) and character device (/dev/rdisk*) file for each storage device. Writing an image to the /dev/rdisknumber character device is faster than to the /dev/disknumber block device.

Example 1. Writing an ISO Image to a Disk

To write the /Users/user_name/Downloads/CentOS-server-x86_64-boot.iso file to the /dev/rdisk2 device:

$ pv /Users/user_name/Downloads/CentOS-server-x86_64-boot.iso | sudo dd of=/dev/rdisk2

Wait for the command to finish. Note that no progress bar is displayed; however, to check the status of the operation while it is still running, press Ctrl+t in the terminal:

4.39GiB 0:03:08 [23.8MiB/s] [============================================================================================================================================================>] 100%
0+71904 records in
0+71904 records out
4712300544 bytes transferred in 188.501782 secs (24998706 bytes/sec)

The speed of the data transfer depends on the speed of your USB ports and the flash drive. After the prompt is displayed again, the data transfer is finished. You can then unplug the flash drive.

The flash drive is now ready to be used as a boot device. You can continue with Booting the Installation on 64-bit AMD, Intel, and ARM systems on AMD64 and Intel 64 systems or Booting the Installation on IBM Power Systems on IBM Power Systems servers.