problem installing Mavericks on MacBook Pro

I have downloaded Mavericks to my MacBrook Pro but the installation process fails. After I have agreed to the license agreement I'm told Mavericks can't be installed on my Mac hard drive because it's used as the backup for Time Machine. Well, it isn't. I have an external hard drive that I use for that, and the external drive is the one I have selected for Time Machine backups. I downloaded and installed Mavericks to my iMac with no problem, which incidentally uses the same external hard drive for backups. What gives?