Clean install OS X Yosemite on your Apple Mac

OS X Yosemite is here and it is in the Mac App Store. Just fire up the App Store from the Dock and you can download it free of charge and upgrade your Mac. Before you do though, take a minute to consider your options. Is it best to upgrade or should you perform a clean install? How do you actually carry out a clean install from the download in the App Store? It is actually quite straightforward, although it does require the use of an external disk, such as a USB drive.

Just think about your disk drive for a minute. It has been accumulating caches, temporary files, clutter and junk for years and then you want to go and put a brand new operating system on it? Yes, it is possible, but surely a clean start with a freshly formatted disk drive is better.

Remove everything from a disk and when you install OS X Yosemite it will be free of the junk that is normally found on old drives and it will run faster and be more reliable. One of the reasons why a new computer is so much faster and more responsive than an old one is because it does not have all the clutter of years of use. It is a fresh start and you can do this with Yosemite too.

1. Use Disk Utility

The first thing to do before you upgrade is to check that the Mac’s disk drive is OK. Errors creep in over time and you don’t want to upgrade a disk that is full of errors.

Go to the Applications/Utilities folder and run Disk Utility. Select the Mac’s disk drive on the left, the boot volume, and click Verify Disk on the right. If errors are found, click the Repair disk button. Then click the Repair Disk Permissions button. Don’t bother with the Check Disk Permissions button, it just wastes time. Go straight to the repair function.

2. Download OS X Yosemite

Start the App Store from the Dock and find Yosemite - it’s right there on the home page and clicking it goes to the download page.

The Yosemite installer will automatically start once the download (5GB) has finished. It is important that you do not continue with the installation. Go to the Install OS X menu and quit. The installer is actually stored in the Applications folder as Install OS X Yosemite.app and you don’t need it right now.

3. Clone the disk

This next step requires an external disk, such as a USB drive. Use a utility like SuperDuper! or Carbon Copy Cloner to copy the Mac's internal disk drive to the external one. Select the internal disk for the source disk to copy and the external disk as the destination.

This step will take a couple of hours, depending on how fast data can be transferred from the internal disk to the external one. (The disk you are cloning to doesn't have to be the same size, it just has to be big enough to fit the data - I have less than 250GB of files, so I'm OK below.)

A nice feature of this is that afterwards you can switch off the Mac and switch back on, or simply go to the Apple Menu and click Restart. As soon as the grey screen appears to show OS X is about to start loading, hold down the Option key until you see a list of disk drives. You can choose the one to start the Mac from.

Start the Mac from the external disk by pressing the left/right arrow buttons to select it and hitting enter. You are now running the clone copy of your Mac from the external disk.

Run Disk Utility again and select the Mac's internal disk drive on the left. I had a Windows partition, an OS X recovery partition and a regular OS X partition. I deleted the lot, created one partition and formatted it Mac OS X Extended (journaled).

4. Install OS X Yosemite

Go to the Applications folder and run Install OS X Yosemite Installer.app. Click Show All Disks and then select the Mac's internal disk drive. Click Install.

5. Finishing off

Yosemite first copies itself to the internal disk drive and then restarts the Mac. It is essential that you hold down the Option key as soon as the grey screen appears indicating the OS X is loading. You will see a choice of two disks to boot from - the external disk and one named OS X Installer. Choose the installer.

From then on it's easy. Just follow the prompts, connect to your Wi-Fi, enter your iCloud password and so on. If there are any reboots, they will proceed normally and you don’t need to select a disk.

The final result is Yosemite on the internal disk and Mavericks, or whatever your previous OS X was, on the external disk. When you boot the Mac, it will start Yosemite, but if you ever need Mavericks, just hold down the Option key when the grey loading-OS-X screen appears and select the external disk.