Choose “Options” and select “GUID” then choose “Apply”.

Now go to the “Partition” tab and under ‘Partition Layout’ select “1 Partition” (you can choose multiple if you want to dual boot OS X Yosemite on partitions contained on the external drive, that’s your call but not what we’re covering here) and confirm the partition.

Choose the “Erase” tab and be sure “Mac OS Extended (Journaled)” is selected as the format, give the drive an obvious name like “Yosemite External”, then choose “Erase” to format the volume – this removes all data from the destination drive, be sure you selected the proper volume here there is no turning back.

Attach the external drive to the Mac and select the newly attached drive from the list of volumes in Disk Utility.

Launch Disk Utility in OS X, found in the /Applications/Utilities/ folder.

The external drive will be formatted for this purpose, meaning it will be completely erased and all data on it will be lost. Make the Destination External Drive / Volume Bootableįirst up is making the external disk not a bootable volume, this is done through Disk Utility.

If you use a slow external flash drive or old external hard disk, don’t be surprised if you encounter many beachballs, making this a very suboptimal experience and by no means representative of Yosemite performance in general.

Again, this is dependent on the speed of the external drive that Yosemite is being installed onto. The last point is important, because the experience very well be much slower than what you’re used to.

Patience for a (likely) slower experience.

OS X Yosemite Dev Preview downloaded and ready to go.

An external drive (USB thumb drive, hard drive, SD card, any external disk) with 16GB of space available or greater (the base OS X 10.10 installation uses about 10GB, and you’ll want some extra space for swap, caches, and test files).Requirements for Installing Yosemite onto an External Disk You can speed test your external drives with free third party tools if you’re not sure if the performance is particularly good or bad, but it’s safe to say that faster read and write speeds is better. If you’re going to try this, aim to use the fastest possible external drive for the best experience, otherwise you’ll find the entire process and overall OS X 10.10 experience to be quite slow running off an external disk. The method described in this tutorial works to install a bootable OS X Yosemite instance onto any external disk, whether it’s a USB flash drive, a generic external hard drive, or whatever other external volume you have.

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