Installing Yosemite on a external 1 GB USB 3.0 drive connected to a late 2012 iMac with 8 GB RAM. Download and install up to point where it says installing and will take 22 minutes. At the end od the 22 minutes, reboots and then asks which disk to install on and then says installing and will take 22 minutes etc. Can not get beyond this point. The files for Yosemite appear to be on the target drive. If I select the external drive as the startup disk, will boot to the internal Mavericks disk instead.Have also reset the NVRAM. What else can I do?