A method of starting-up a computer system includes accessing a second storage area of a storage in which program data are stored; loading and executing the program data from a second storage area; mounting an external storage medium connected to the computer system, wherein a file system key that decrypts a file system data is stored on an external storage medium, wherein the file system key is encrypted on the external storage medium; loading the encrypted file system key from the external storage medium into the computer system; decrypting the encrypted file system key by a key stored in the second storage area; setting the decrypted file system key in a cryptographic module established by the start-up process; and decrypting and loading file system data of the encrypted file system by the cryptographic modules by the set file system key, whereby the computer system is started up completely.