Can't wipe HD on old MacBook Pro

[Walter Sobchak]

I've just bought a new MBP Retina Display and want to wipe my old and rather tired MBP to give to a friend.
When I go into the boot up Disk Utility by holding the Command + R Key and click on earsing the hard drive, I get a warning come up which says unable to unmount drive/wipe drive and can't see any way around it.

The drive has been partitioned as I was using Boot Camp but I've already Wiped the Windows partition but it still exists.
Is there any way around this?, I just want to wipe the whole thing and put a fresh OS on it.

[Walter Sobchak]

Thanks for the advice, where would I get a Bootable version of OSX from?, I think I've still got the old CDs mine came with but that is Leopard or Snow Leopard.
Only thing I have is time machine backups on my external HD which isn't going to help as I don't want to restore the system.
I'd also like to revert it back to Mavericks if possible? as Yosimete ran really badly on it.

[Walter Sobchak]

I found my old Leopard Install CD and managed to boot it from that, seems to be working now.
The issue before was there is a very small partition OSX makes to boot from so I couldn't erase the HD.