So I've been stuck on this question for a very long time.
How do you use double encryption to encrypt a hard disk so that all of the following conditions are met:
All data on the hard disk is encrypted.
There are multiple users and each user has his own key to make the harddisk readable/writable. Think for example of a normal user and the administrator. (everyone can access any file though, no permissions needed)
If a user forgets his key, a new key can be generated for him, with which he simply has access to the stored data.