If you are using a new harddisk (or want to wipe out the whole partition table of your disk), a new partition table needs to be created. The “Guided partitioning” does this automatically, but when partitioning manually, move the selection on the top-level entry of the disk and hit Enter. That will create a new partition table on that disk. In expert mode, you will then be asked for the type of the partition table. Default for UEFI-based systems is “gpt”, while for the older BIOS world the default value is “msdos”. In a standard priority installation those defaults will be used automatically.

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	When a partition table with type “gpt” was selected (default for UEFI systems), a free space of 1 MB will automatically get created at the beginning of the disk. This is intended and required to embed the GRUB2 bootloader.

If you have an existing other operating system such as Windows and you want to preserve that operating system while installing Debian, you may need to resize its partition to free up space for the Debian installation. The installer supports resizing of both FAT and NTFS filesystems; when you get to the installer's partitioning step, select the option Manual and then simply select an existing partition and change its size.

While modern UEFI systems don't have such limitations as listed below, the old PC BIOS generally adds additional constraints for disk partitioning. There is a limit to how many “primary” and “logical” partitions a drive can contain.

On such old PC BIOS system, “primary” partitions are the original partitioning scheme for PC disks. However, there can only be four of them. To get past this limitation, “extended” and “logical” partitions were invented. By setting one of your primary partitions as an extended partition, you can subdivide all the space allocated to that partition into logical partitions. You can create up to 60 logical partitions per extended partition; however, you can only have one extended partition per drive.