Going back from Yosemite to Snow Leopard

Hi,

I know similar questions have been asked to death, but I have read all the instructions on the web and still having problem with this.

First of all, my laptop is a macbook pro 15" late 2008/early 2009. And it still has the original HDD drive it came with.

Secondly, due to the fact that I dropped my laptop a couple of years' back, the DVD player doesn't work, so I've made a usb installer for Snow Leopard.

So to go back to Snow Leopard I followed the instruction of first erasing/partitioning my disk by going to Recovery. First problem occurred because I couldn't partition or erase the main hard drive (not the macintosh HD, but the parent folder which is the Toshiba or something, I can't remember now). I got the "could not unmount disk" error. So I erased the macintosh HD instead. That went smoothly. Then restarted with the option key again and selected the Snow Leopard installer. Everything goes well until when I have to choose the volume in which I have to install, the only available option being the macintosh HD, but I get an error that macintosh HD can't be used to startup from. So I went back to Recovery (and under the Recovery it says 10.10 recovery, so I guess Yosemite isn't completely gone) I have an option for reinstalling OS X, which of course is Yosemite, which seems to be working fine for now (although it's taking 5 hrs). So I'm going back to Yosemite again.

So I'm wondering what the problem is? Is my hard drive damaged (perhaps from when I dropped it a few years ago)? Or there is some other way that I can try this? I really can't go back to Yosemite, nothing works there, and this is my work computer!

Thank you for the help, and for reading this long message to the end!