Why you might want multiple keys for disk encryption
For a moment let's go back to the commentator's suggestion for recovering from key loss from my first entry on full disk encryption:
Many systems support multiple keys (e.g. dm-crypt). Just keep one sealed in a safe.
One question you might ask here is why you need (or want to use) a second key for this backup purpose. After all, you can put a copy of your single key in the safe, and it works just as well to decrypt the data (and if you use only a single key you might avoid the multi-key data loss issues).
After thinking about it a bit, the answer I've come up with is that using a separate backup key is safer in certain sorts of real world scenarios, because it lets you safely change your normal key in the field.
Suppose, hypothetically, that you are on vacation with your laptop and you believe that your normal disk encryption key has been compromised; you've been watched while you typed it, you've found a keylogger on your machine, or whatever. You need to immediately change your key in order to maintain security, but if you only have a single key changing your normal key also invalidates the backup key you have locked in your safe at home. With multiple keys and a separate backup key you can change, revoke, or scramble your normal key while still being able to recover your data if the worst comes to the worst.
(In more extreme situations you can revoke or scramble your normal key, the only one you have memorized, and then be able to honestly say that you have no ability to decrypt the disk right now. The other people involved might even believe you.)