Last week, we showed you how to keep a physical copy of your Mac App Store digital download of OS X 10.10 Yosemite in the form of a bootable install USB drive, which comes in handy if you want to do a clean install of the operating system. Now, we’ll show you how to perform a clean OS X Yosemite install on your Mac in the following step-by-step guide (via OSXDaily).
Make sure you backup all your data before proceeding as the process requires formatting the Mac hard drive. Also, keep the OS X Yosemite bootable installer drive handy before you begin.
Insert your OS X Yosemite bootable USB drive into your Mac and reboot the system.
While rebooting, hold down the Option key till the following boot selection menu appears.
Select “Install OS X Yosemite” as the drive to boot and hit Enter.
Now you’ll be presented with the “OS X Utilities” screen. Select “Disk Utility”.
Choose the hard drive or partition from the left side menu which you want to perform a clean Yosemite install, and then select the “Erase” tab. Choose the Format type as “Mac OS Extended (Journaled)”, name the drive (e.g. Mac OS) and then click “Erase”.
Once the drive has finished formatting, quit Disk Utility to return to the OS X Utilities menu and select “Install OS X” as shown below.
Finally select the freshly formatted hard drive / partition to set as the destination for the fresh install of OS X Yosemite. Simply follow the onscreen instructions to complete the install.
That’s it, enjoy!
If you have any questions, feel free to ask in the comments section below.