HOW TO |MAKE A BOOT DISK
1 Format the disk
Plug in the drive on which you’ll install El Capitan and wait until it mounts. Press ß+ç+u in the Finder to open the Utilities folder, open Disk Utility and select the drive on the left. Click the Partition tab, click on the partition layout diagram, select ‘1 Partition’ from the drop‑down menu above it. Click the Options button below the diagram and ensure GUID Partition Table is selected; it should be, and it’s important that it is in order for the disk to be bootable. With the partition selected in the diagram, select ‘Mac OS Extended (Journaled)’ as its format on the right, and name it. Now click Apply to prepare it for El Capitan installation.
2 Begin installation
Now download the installer, as described under Preparation on page 51. The installer will run as soon as it has downloaded. When you reach the point where it asks you where to install El Capitan, click Select Disk and choose the partition on the drive you just prepared, and click OK. Click OK again to begin installing. You’ll see a message telling you that all applications will quit and your Mac will restart several times. Now, wait. If you’re installing on a USB 2.0 hard drive, the process could take quite a while. When it’s finished writing the necessary files to disk, your Mac will restart in El Capitan to complete the installation.
Set up OS X El Capitan
At this stage, you’ll need to go through the process of setting up the OS from scratch as it’s a brand new installation. So, you’ll need to provide your country preference for the keyboard settings, allow or disallow location sharing, and sign in to your iCloud account if you want to use it with the El Capitan beta. You’ll also be told about the Feedback Assistant in the beta. This is a special app included with Apple’s beta operating systems that allows you to send details of problems you have to Apple. It’s installed when you take part in the beta program, and you sign into it using your regular Apple ID and password.
Switch between versions of OS X
The final step is to provide a name and password for the admin user account on your El Capitan beta. The system will then create the user account, which can take quite a while on a slow drive as there are many files to create. Once done, you’ll be running El Capitan. Have a poke around, discover what’s different, and use the Feedback Assistant to notify Apple of problems. When you want to use your usual version of OS X, go to System Preferences > Startup Disk and select it, then restart; this sets it as the system that’s used every time you power on your Mac. Alternatively, you can hold å at the startup chime to make a temporary choice.