I've made a partition on my hard drive, so I currently have Macintosh HD (call it 1) and Macintosh HD 2 (call it 2).
I downloaded the Yosemite installer with my Mavericks install on 1, then ran it and selected 2 as the disk to install on. It worked fairly well and I'm running Yosemite without any problems.
But tonight I tried to switch over to Mavericks, so I rebooted and held down option. When the list came up, it contained recovery volumes for OS X 10.9 and 10.10. It had Macintosh HD 2 on it (my Yosemite partition), but not Macintosh HD. Instead of my Mavericks partition, it showed an 'Install OS X' volume. Selecting this shows the Apple logo with a progress bar under it, then boots into Yosemite.
I can view /Volumes/Macintosh\ HD/ and see that everything is still there.
How can I boot back into my Mavericks install?