How to Clean Install OS X Yosemite

Most Mac users who are upgrading to OS X Yosemite should go through the standard process via the Mac App Store and start the installation from there, it’s by far the easiest approach and it generally goes flawlessly. Some users like to perform what is known as a clean installation of Mac OS X though, which is essentially wiping the Mac drive, and starting fresh with a brand new untouched install of OS X Yosemite, and then manually copying or accessing files and re-installing apps. While clean installing is not particularly difficult, it’s considered more advanced because it’s a much more hands-on and lengthy process than simply updating to OS X 10.10 through the App Store.

For those who are interested in performing a clean install on their Mac, which basically makes OS X Yosemite appear as if it was brand new, without any settings, preferences, apps, or anything else carried over from before, we’ll cover the process thoroughly in this walkthrough.

The requirements for fresh installsare fairly straightforward:

• A Mac which can run OS X Yosemite – in terms of system requirements, the machines which support Yosemite are the same as that which can run Mavericks
• An OS X Yosemite bootable installer drive – usually this is a USB disk but it can be whatever so long as the drive is bootable, the installer by itself is insufficient unless you’re fresh installing onto a different partition or volume. If you haven’t made one of these yet, do that first

Important: A clean install of OS X Yosemite requires formatting the Mac hard drive, meaning all contents on the disk will be removed and erased. Be sure you have your important files backed up before attempting this.

How to Perform a Fresh Installation of OS X Yosemite on a Mac

1. Back up the Mac with Time Machine or by manually copying your most important data to an external drive – never skip backing up your important data
2. Attach the OS X Yosemite bootable installer drive to the Mac, then reboot as normal
3. During boot down the OPTION key until a boot selection menu appears, choose “Install OS X Yosemite” as the drive to boot (this is the USB installer)

4. When the “OS X Utilities” screen appears, choose “Disk Utility”

5. Choose the hard drive (or partition) from the left side menu which is the destination for the clean Yosemite install, then choose the “Erase” tab
6. Choose the Format type as “Mac OS Extended (Journaled)”, name the drive something obvious (like Macintosh HD or Yosemite HD), and then choose “Erase” – this will format the drive / partition and all contents on it

7. When the drive has finished formatting, quit Disk Utility to return to the OS X Utilities menu
8. Now from the “OS X Utilities” menu, select “Install OS X”

9. Click through the Terms of Service and basics, and select the freshly formatted “Macintosh HD” or “Yosemite HD” drive to set as the destination for the fresh install of OS X

OS X Yosemite will install a completely clean slate version of itself on the blank destination drive, giving the user a fresh start. Because the drive was formatted first, no existing operating system will be updated, and no other contents on the drive will be brought over.
As covered in a separate article, installing the newest version of OS X can take quite a while, though clean installs are generally faster than upgrades. Nonetheless, the Yosemite installation may appear stuck, but it’s probably not, and waiting it out is usually the right choice. 
When installation has complete, the Mac will reboot into OS X Yosemite and go through the normal new setup process. 

Remember, no data will be on the volume aside from the fresh Yosemite installation, so it’s up to the user to re-install necessary apps, adjust preferences, and most importantly, recover or transfer over again their important documents and files.

Ref:http://osxdaily.com/2014/10/18/clean-install-os-x-yosemite/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed:+osxdaily+(OS+X+Daily)&utm_content=FaceBook