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As you're now aware, you can boot into the Mac's Recovery HD partition, to reinstall a copy of OS X. The difficulty here is that you must re-download the entire install, which can be multiple gigabytes. If you have a slow internet connection, this can take a long time, or if you have a data cap on your internet connection, it may end up costing you money. Wouldn't it be more convenient if you could create your own startup disk that included the OS X installer? Well, you can, and I'm going to show you how.
First thing you need is a USB stick that has eight gigabytes of storage or more, and I have one with me. So, I'll plug that into my Mac and it appears on the desktop. Now, let's launch Disk Utility, and here's my USB stick. Now, I have to format it so it can become bootable. So I'll click on Partition. I'll choose one Partition. Click on Options and select GUID Partition Table and click OK. The format will be Mac OS Extended (Journaled), and I'll call this Install OS X, then I click on Apply.
Indeed, I do want to Partition it, and now it's properly formatted. For now, I'll leave Disk Utility open because I'm going to need it in a second. When we downloaded a copy of OS X, I asked you to keep a copy of the installer, and here's why. We'll go to Documents, and here's my installer. It's still here because I moved it to another folder. Now, I'll Control-click on it, and choose Show Package Contents. Open the Contents folder and then open the SharedSupport folder, and this is the file I'm looking at.
This is the OS X installer, so I'll move this window aside. I click on Restore. I'll choose as my source, this InstallESD.dmg file. The Destination will be my OS X USB key drive. Now, I click on restore. It will erase it, asks for my password, and OK. As you can see, this takes awhile to do and we're not going to make you sit through this whole thing.
We'll rush through this a bit. And the job is done. We now have a USB eight gigabyte key drive, that not only includes the OS X installer, but is bootable as well. So, we can take a look at its contents. Here's the installer, and here are the things necessary to boot the Mac. So, to boot this Mac from this drive, you could go to System Preferences, Startup Disk, select the drive and then choose Restart. Or if your Mac won't boot up because the OS is corrupt in some way, you can simply insert that drive into a USB port on your Mac, restart the Mac, and hold down the Option key.
When you do, you should see this volume appear among your choices to boot from. Choose to reinstall Mac OS X, and instead of downloading it from the internet, it will use the copy that's stored on the USB drive. It's a much faster installation, plus it makes a great Emergency Startup Drive. And that's how to make a bootable installer for your Mac.
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