Rozdělením disku se na disku vytvoří několik vzájemně nezávislých oddílů (angl. partition). Každý oddíl je nezávislý na ostatních. Dá se to přirovnat k bytu rozčleněnému zdmi — přidání nábytku do jedné místnosti nemá na ostatní místnosti žádný vliv.

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.

Obecně změna oddílu, na kterém je souborový systém, znamená ztrátu dat, takže si raději disk před změnami do tabulky diskových oddílů zazálohujte. Podle analogie s bytem a zdmi, z bytu také raději vynesete veškerý nábytek, než budete přestavovat zdi.

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.

To losslessly resize an existing FAT or NTFS partition from within debian-installer, go to the partitioning step, select the option for manual partitioning, select the partition to resize, and simply specify its new size.