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. Th