Yosemite won't install - Time Machine conflict

I'm attempting to install Yosemite on a late 2008 aluminum MacBook, which is currently running O.S. 10.6.8. When I go to click the disk where the program should be installed, I get a message, "This disk is used for Time Machine backups", and the installer refuses to let me select the disk. I do have a separate disk that is used for Time Machine backup.