AFAIK, our current disk encryption methods (dm-crypt) preserve the key (as well as decrypted data) in RAM while the computer is running with an implicit assumption that the memory is volatile and key will be lost along with decrypted data when computer is shut down.
However, future advancements in technology may make volatile memory obsolete. Given operating-systems adapt to work without RAM in future, what are the ways to achieve disk decryption without volatile memory? Is it possible, or will we always need volatile memory for cryptography?
After thinking about this, I conclude that there are inherent properties in cryptography that necessitate the presence of a temporary insecure location. I don't think it is even theoretically possible to have algorithms that need to access the key once and be done with it without storing it in an insecure location. And that volatile insecure location also acts like a kill-switch.
(*insecure = not encrypted).