Can not download OS X Yosemite (Hard drive being used for time machine back up message)

Hello to All,

I attempted to install osx yosemite last night. Everything was going well until it came to select the hard drive. It says that the main hard drive is being used for time machine back ups. I have a separate external hard drive for time machine back ups. I tried to disconnecting my external hard drive and i get the same message. I have my main internal hard drive replaced in my imac several months ago because it crashed. Im not sure if this has anything to do with it. Any help would be greatly appricated.

Thanks