I understand that guesses per second depends on the hardware and the encryption algorithm, so I don't expect an absolute number as answer.

For example, with an average machine you can make a lot (thousands?) of guesses per second for a hash created with a single md5 round, because md5 is fast, making brute force and dictionary attacks a real danger for most passwords. But if instead you use bcrypt with enough rounds, you can slow the attack down to 1 guess per second, for example.

1) So how does disk encryption usually work?

This is how I imagine it, tell me if it is close to reality: When I enter the passphrase, it is hashed with a slow algorithm to generate a key (always the same?). Because this is slow, brute force is not a good approach to break it. Then, with the generated key, the disk is unencrypted on the fly very fast, so there is not a significant performance lose.

2) How can I test this with my own machine? I want to calculate the guesses per second my machine can make.

3) How many guesses per second are possible against an encrypted disk with the fastest PC ever so far?