As part of a custom installation, you are allowed to manually configure your disks. This menu handles disk partitioning -- splitting up disks into various sections for different uses (customarily for different operating systems, or different logical disks for the same operating system.) Mastodon uses a fairly simple partitioning scheme -- it breaks a disk up into a 128mb swap partition and reserves the rest for the root partition. (On large disks, it also creates a 40mb /boot partition at the start of the disk to hold the Linux kernel.) If you don't want this exact arrangement, you should do your own partitioning.

To partition a disk, select it in the menu and press [return] (or double-click it with the mouse, if you have one). This starts the cfdisk visual disk partitioning program, which you may then use to arrange your disk. When you are finished with partitioning all the disks you want, press [next] to proceed to setting up where you wish to have these partitions mounted.