My early 2009 iMac with Mavericks has one HD, which I selected after starting the install process, as the destination for the Yosemite OS. The window then disappears, and it appears that nothing happens after that point. I waited several hours in case there was a background process happening, to no avail. Rebooted, and tried again: same result. What's happening? I feel as though I may be missing something obvious, but all previous OS installs have been very straightforward.