Although 3 years old, this is a good article and a link to a paper about coldboot attack against full disk encryption technology.

In a nutshell, it is related to data not being encrypted when stored in RAM and although it is volatile: “from 2.5 to 35 seconds to reach a Null State” when switched off, it can be recovered with a few techniques such as dropping the RAM temperature to slow down that “null state” or booting up the device through a very small kernel OS so only a small portion of the RAM is over written through a USB device for example.

What makes this attack even more powerful is that a lot of information “derived from the encryption keys” are stored in RAM, usually to speedup calculations.
The author then warn those attacks would be very difficult to prevent without a radical change in hardware architecture or “overhaul of the encryption process itself”.

Arstechnica ARTICLE