Q: cannot update my OS?

When i try to update my OS to os x yosimite it downloads the installer to my macbook pro 17'' model: A1297 and when i go into the installer it asks me to select a drive to download the OS to and the only one it shows is a time mac 250 gb hd and it says you cannot save to this hard drive it is used for backups only? sooo what do i do?!?!:[ ide like to keep my computer up to date with the latest stuff! but it will not let me do anything.. its currently running on os x 10.8.5

system specs are

2.66 ghz intel core i7 processor

memory: 8gb 1067 mhz ddr3

graphics: nvidia GeForce GT 330m 512MB

software : OS X 10.8.5 (12f45)

and its a mid 2010

any help would be greatly appreciated thank you