I just replaced the HDD on my Macbook mid-2010 7.1 with a SanDisk Ultra II 2.5" 480GB SATA Revision 3.0 SSD. I created a bootable USB drive loaded with El Capitan. I powered up holding down the Option key but nothing happens. This is a machine I bought used and don't have any install disks. I inserted the Snow Leopard CD from my iMac and booted again. The only option is INSTALL OSX. When I click on the icon, nothing happens. Any suggestions would be most welcome.