New SATA internal drive won't boot?!

I've just bought a Western Digital 2.5 inch Scorpio Blue 750GB 5400rpm SATA 3GB/s 8MB Cache Internal Hard Drive for my mid 2010 macbook pro which was running mountain lion. At the same time, I bought some new RAM (upgrade from 4 to 8G which works fine using my original hard drive). The new drive won't boot at all and when I insert the original snow leapoard disc (it shows an apple on the screen and looks like it'll work but then goes to kernel panic). What could the problem be??