Q: Cant upgrade macbook pro to yosemite

I am trying to update my macbook pro which is running Snow Leopard.

I have time machine setup to backup to an external hard drive and this appears to be doing the right thing.

I have been through the process of downloading the Yosemite upgrade but when I get to the install pane I am shown the only hard drive available is the internal hard drive and I cannot continue as I am told this drive is being used for Time Machine backups. I am unable to get any further than this point.

Can someone tell me what to do please?

Thanks