When I upgraded to Yosemite, the installer offered to painlessly switch my hard drive to use FileVault 2 full disk encryption. I said yes. Unfortunately, the OS didn’t generate the recovery key that it was supposed to. I was left with an encrypted disk, and no recovery key I could file away in case I forgot my password. After some investigation, I discovered that you can ask OS X to create a new recovery key for you from the command line.