Unable to select my hard drive to install Yosemite

My elderly iMac is currently running OSX 10.6.8. I believe I am eligible to upgrade to Yosemite, but when I try it gives me this message: This disk is used for Time Machine backups. I have been backing up my files to DropBox, and have never used Time Machine. Time Machine is turned off, and always has been. I do not have a separate hard drive connected.

Would I have more luck upgrading to Mavericks? I really didn't want to go through this process again...

I have 869 GB space available, so I don't think that is the problem.

Any ideas?