i have an iMac with two external HDs. on one of the HDs i upgraded from mavericks 10.9.5 to yosemite. everything worked fine. then i needed to scan something with a scanner that is not supported beyond 10.6.8, so i went to the startup discs and selected the 10.6.8 system on the internal HD. after the scan i wanted to go back to yosemite, but it no longer shows up as a startup disc. disc utility says the yosemite partition is fine. appreciate any ideas.