How to Install OS X Yosemite on Any External Drive (Thumb Drive, USB Disk, etc)
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.