Why does GParted force 1Mb "Free Space Preceeding"?
The reason for GParted to reserve 1MB free space preceding when creating the first partition in your USB stick is because an MBR partition table must be placed in order to store information about the partitions created on the USB stick.
A classical generic MBR includes:
- 446 bytes of bootstrap code area;
- 4 partition entries, each occupys 16 bytes; and
- 2 bytes of boot signature.
Since GParted (and most partition editors) supports MB division only, it will choose to reserve 1 MB space for storing the partition table – which after creating the first partition, the preceding space won't be shown up in the GParted GUI.
Reasons why GParted forces 1Mb “Free Space Preceeding”
In addition to partition alignment (by mibibyte) the drive space behind the first 512 bytes within the first mibibyte is 'reserved' for additional data of the grub bootloader for BIOS mode in an MSDOS partition table.
In a GUID partition table the corresponding data are stored in a small partition (1 MB is enough) with the bios_grub flag (and without any file system).
See this link: DiskSpace | Ubuntu Community Help Wiki