OS X Yosemite install error / Harddrive is used for backups? I'm trying to install Yosemite onto a 15" Macbook Pro / Late 2008 model with OS X Mountain Lion on it.  I've checked Apples site and it says I can install Yosemite on this machine with these specs. When I click through the installer and get to my harddrive it won't let me select it and I get an error message that reads "This Disk is used for time machine backups"  which it isn't. I have an external drive dedicated just for that purpose. I do not have the external connected when I try to install and I have time machine turned off in preferences. Also, my harddrive is not listed in the drive list in time machines preferences.  I have no idea what's going on. I apologize if I'm missing something obvious. Can someone please help me?
thank you,
Justin
I had the same problem. Turned out it had once been used for backups. I removed the folder Backups.backupd in the HD root and everything went fine after that. I assume you have solved it, but I'm writing this in case somebody else have the same problem.

It's unlikely you'll need to go though those complicated steps in the answers above.