To avoid issues installing Yosemite on a partition, I thought I'd try an external drive, which I assumed would be safer. I mounted the external (FireWire through a Thunderbolt adapter) and ran the installer from my iMac's internal fusion drive. The installer hung with 14 minutes remaining. After an hour or so, I quit, figuring that I could reboot off the internal after selecting it as the startup drive. However, every time I restarted, the Yosemite installer ran. The only fix I found was to reboot in recovery mode and reinstall Mavericks on the internal drive. That let me restart in 10.9, but took a long time. I've since read that the Yosemite installation may take hours, so I may just not have waited long enough. But if I run into the same problem again and want to abort the external installation, is there any way to reboot off the internal drive in Mavericks without reinstalling the OS?