The key used for these kinds of symmetric encryption is called as File Encryption Key (or FEK). This FEK is in return encrypted with a public or a private key algorithm like RSA and stored with ...

I rebooted. Logged in, and there's some notification asking me to backup the key because it is encrypting my file system. I didn't ask it to do that, and I wasn't even logged in as admin before ...

Encrypting files ... while someone could get at your files if they know your password (the decryption key), they won't be able to take out a drive from your system and access what's on it ...