The BIOS firmware and MBR partition tables in DOS and Windows PCs requires that an extended partition be used when more than four drives are assigned to the same physical disk. After the extended partition is created, "logical drives" are formatted, and drive letters are assigned (F: and G: in the example below). See primary partition
and basic disk
An Extended Partition Allows More Drives
An extended partition allows for more than four drives on the same physical disk.