Partitionering af din disk refererer til det at opdele din disk i sektioner. Hver sektion er så uafhængig af de andre. Det svarer cirka til at placere vægge i dit eget hus; hvis du tilføjer møbler i et rum, så påvirkes de andre rum ikke.

If you already have an operating system on your system (Windows, OS/2, MacOS, Solaris, FreeBSD, …) which uses the whole disk and you want to stick Debian on the same disk, you will need to repartition it. Debian requires its own hard disk partitions. It cannot be installed on Windows or Mac OS X partitions. It may be able to share some partitions with other Unix systems, but that's not covered here. At the very least you will need a dedicated partition for the Debian root filesystem.

You can find information about your current partition setup by using a partitioning tool for your current operating system, such as the integrated Disk Manager in Windows. Partitioning tools always provide a way to show existing partitions without making changes.

Generelt vil ændring af en partition med et allerede eksisterende filsystem ødelægge enhver information på partitionen. Du bør derfor altid lave sikkerhedskopier før udførelse af en ny partitionering. Du kan overføre denne analogi til dit hjem. Det vil være smart at flytte alle møbler væk før en væg flyttes, ellers risikerer du at ødelægge dem under arbejdet.

Several modern operating systems offer the ability to move and resize certain existing partitions without destroying their contents. This allows making space for additional partitions without losing existing data. Even though this works quite well in most cases, making changes to the partitioning of a disk is an inherently dangerous action and should only be done after having made a full backup of all data. For FAT/FAT32 and NTFS partitions as used by Windows systems, the ability to move and resize them losslessly is provided both by debian-installer as well as by the integrated Disk Manager of Windows.

For at ændre størrelse på en eksisterende FAT- eller NTFS-partition uden datatab inde fra debian-installer, så gå til partitioneringstrinnet, vælg indstillingen for manuel partitionering, vælg partitioneringen der skal ændres størrelse på og specificer den nye størrelse.