Where is the disk key? Is it stored in the main board, or in the flash disk itself? E.g. if you have to replace the motherboard, do you lose the data, or becaus

Where is the disk key? Is it stored in the main board, or in the flash disk itself? E.g. if you have to replace the motherboard, do you lose the data, or because of being in the flash disk it is OK? BTW, if being in the flash disk, how secure it is the key handling?