PC3L will POST, won't boot

Having bought a new laptop I thought I'd be smart, max out its RAM and then "trickle down" the modules I took out of it into my older laptop that I use as a workstation.

Now, my new laptop is an Alienware 15 (2015) and my older laptop is a Dell Studio 1749.

The AW came with 2x4GB modules and I was running 1x4GB and 1x2GB in the studio. No problems.

Replaced the AW's memory and fitted it to the studio. When I try and boot it it POSTs OK, but windows doesn't even flash it's loading screen...

PC3L will POST, won't boot

PC3L will POST, won't boot