How to create a bootable usb for EI captain?

Howdy Guoxuan,

When you mention "createinstallmedia" it sounds like you would like to create a bootable installer for El Capitan.

Note "createinstallmedia" is not a built in function but is part of the installer package as shown in this article.

Create a bootable installer for OS X - Apple Support

Use the 'createinstallmedia' command in Terminal

1. Download the OS X installer from the Mac App Store. Quit the installer if it opens automatically after downloading. The installer will be in your Applications folder.
2. Mount your USB flash drive or other volume. You could also use a secondary internal partition.
3. Open the Terminal app, which is in the Utilities folder of your Applications folder.
4. Use the

   createinstallmedia

   command in Terminal to create the bootable installer. Examples of this command are in the next section. For detailed usage instructions, make sure that the appropriate Install OS X app is in your Applications folder, then enter one of the following paths in Terminal:

Path for El Capitan:

/Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia

Note in step 4. the appropriate installer is expected to be in the Applications folder so you can get to it.

The article further gives a more complete example of the command.

The following examples assume that the OS X installer is in your Applications folder and the name of your USB flash drive or other volume is MyVolume:

Example for El Capitan:

sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app

If instead you are just trying to create a bootable USB drive, then you would just install El Capitan on that drive as normal, selecting the USB drive as the destination.

Upgrade to OS X El Capitan - Apple Support

Best regards.

Nubz N provided the Apple link for the instructions and Terminal command that's used.

Make sure the Install OS X El Capitan.app file is in your /Applications folder. And you have an 8GB or greater USB thumb drive that is formatted as OS X Extended (Journaled) with the GUID partition table. When using the command below, replace MyVolume with the volume name of your USB thumb drive (e.g. Untitled). Run the command below from an account that has administrator rights and post your results here so we can see what may have gone wrong.

Open Terminal and copy/paste the following replacing MyVolume with your USB thumb drive volume name:

sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app

Here are my results. It take awhile to run:

administrator$ sudo /Applications/Install\ OS\ X\ El\ Captian.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ El\ Captian.app

Password:

Ready to start.

To continue we need to erase the disk at /Volumes/Untitled.

If you wish to continue type (Y) then press return: y

Erasing Disk: 0%... 10%... 20%... 30%...100%...

Copying installer files to disk...

Making disk bootable...

Copying boot files...

Copy complete.

Done.

You are welcome. Thanks for the update.