You need a backup software like Nero Backup. To span multiple DVD’s the software is required to encode how it is spanned and the same software will be the only one that you can use to restore that data.
The best way to backup your 50 gigabyte hard disk drive into various DVD is to create an ISO image and split it into 4 gigayte each since DVD has the capacity to store up to 4.7 gigabytes. You can also manually copy and paste files but it is not recommended. Another possible remedy is to use an third party sofware tools like Acronis True Image to create an image for the entire hard disk drive.