Yosemite’s download-only shtick is revolutionary and, in most cases, incredibly convenient. But sometimes, you’ll miss having a physical disk with the installer on it. You might wish you had one if, for example, you want to install Yosemite on a huge array of Macs, like the ones at a school or in a business; downloading that 5-gig installer over and over again would drive you crazy.
You might also wish you had an installation disk when your Mac starts acting up and the tricks described in Appendix B don’t seem to work.
As it turns out, you can make a Yosemite installation disk fairly easily. All you need is a hard drive, a flash drive, or even a blank DVD. (Hard drives and flash drives are much faster at performing the actual installation.)
If you’re planning to use a flash drive or a hard drive, the first step is to format it so that the Mac will be able to start up from it. To do that, connect or insert the drive. (It needs at least 5 gigabytes of free space. These steps assume that there’s nothing else on it.)
Now open Disk Utility (it’s in your Applications→Utilities folder). In Disk Utility, click the drive’s name in the left-side list (not the partition name beneath it). Then click the Partition tab in the main window.
Under Partition Layout, choose 1 Partition. Then click the actual partition—the big rectangle “map” in the middle of the window—and then click Options beneath it. In the resulting dialog box, click GUID Partition ...