Trying to upgrade from 10.6.8 to Yosemite 'file system verify or repair failed'

I upgraded from 10.6.8 to yosemite and during the installation process I get a message 'file system verify or repair failed'.

I don't have any original system discs, so if I use the Disc Utility in the installation screen can I reformat and some how reinstall a fresh yosemite system? Unless there is another way to repair the disc in the current screen.

I'm a little bit stuck, I've read quite a bit about repairing the disc but I repaired one disc and another had the repair button greyed out. I'm not to sure how the partitioning works in the left column on the disc utility window.

I've also read that I need to return to Snow Leopard to repair the disc but have no idea how to do that. I do have a spare hard drive if needed.

Any help would be very much appreciated. Thanks.