We’ve covered how to install OS X Yosemite onto a separate partition of an internal hard disk and dual boot with OS X Mavericks, but many Mac users have wondered if it’s possible to install OS X 10.10 onto an external drive instead, thereby avoiding any modifications to the primary Mac hard drive or partition table. The answer is yes, you can install Yosemite onto an external disk, and yes it will be bootable, we’ll show you exactly how to do that.



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. 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. 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.

Requirements for Installing Yosemite onto an External Disk

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)

A Yosemite compatible Mac

OS X Yosemite Dev Preview downloaded and ready to go

Patience for a (likely) slower experience

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

Make the Destination External Drive / Volume Bootable

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

Launch Disk Utility in OS X, found in the /Applications/Utilities/ folder Attach the external drive to the Mac and select the newly attached drive from the list of volumes in Disk Utility 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 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 “Options” and select “GUID” then choose “Apply”

You’re now ready to install OS X Yosemite onto the external disk drive, whether it’s a USB hard disk, a flash drive or thumb drive, or whatever else you want to use. Just remember it’s going to be formatted.

Install OS X Yosemite onto the External Disk Drive

Now you’re ready to install OS X Yosemite onto the newly made bootable external disk. We assume you already have the OS X Yosemite downloaded and ready to go, if not you can get it from the Mac App Store through the Mac Dev Center.

Launch the OS X Yosemite installer as if you were going to install as usual Agree to the EULA and licensing terms, then choose “Show All Disks” to find the external volumes Select the external volume to install Yosemite onto and choose the “Install” button, enter the admin password when requested and the Mac will automatically reboot to begin the installation process as usual

Installing onto an external drive may take a while, depending on the speed of the destination disk. Speaking of speed, it’s hard to overstate that performance of Yosemite (or OS X, or any other OS for that matter) running off an external volume is often considerably slower than running off an internal volume, and hardly representative of the true native experience of running on an full speed drive. Performance ultimately depends largely on the speed and connection of the external drive being used, but something like an USB 2.0 external drive is undoubtedly much slower than whatever drive is built internally into the Mac. On the other hand, a fast Thunderbolt or USB 3.0 drive may perform quite well for testing purposes.

Enjoy OS X Yosemite on your bootable external drive!

For those wondering, in creating this walkthrough I installed Yosemite and ran it off of a fairly generic ‘fast’ USB thumb drive. While performance is sufficient to get a feel of the general changes and appearance, the overall experience is tremendously slow when compared to running OS X Yosemite directly off of an internal SSD. Thus if you’re actually looking to test OS X 10.10 and certainly if you’re looking to develop for it, do yourself a favor and install it onto a fast drive or a separate partition instead.