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The Raspbian Stretch OS root partition should come as 3 GB. How can I make root as the 3 GB partition and the rest as a free partition.

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There are some quirks with adding a third partition to the Raspbian image. First it will expand its root partition on first boot to the whole SD card so you cannot simply add a partition afterwards. Second is that using a disk partitioner like fdisk (I prefer parted) it will silently modify the PARTUUID to a unique id. Because Raspbian uses the PARTUUID in cmdline.txt and in /etc/fstab to address partitions, it will fail booting then. For further information look at [1]. So we have to expand the root partition to 3 GB, add the third partition and modify /boot/cmdline.txt and etc/fstab before the first boot. For addressing the partitions we use /dev/mmcblk0pX.

Let's do it. First flash an image, assuming the SD card is at /dev/sdb. For reference I use Raspbian Stretch Lite 2018-04-18.

pc ~$ sudo -Es
pc ~# unzip -p 2018-04-18-raspbian-stretch-lite.zip | dd of=/dev/sdd bs=4M conv=fsync
pc ~# parted /dev/sdb print
Model: Mass Storage Device (scsi)
Disk /dev/sdb: 8069MB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags:

Number  Start   End     Size    Type     File system  Flags
 1      4194kB  49,4MB  45,2MB  primary  fat32        lba
 2      50,3MB  1862MB  1812MB  primary  ext4

pc ~# parted /dev/sdb resizepart 2 3050,3MB   # expand root partition
pc ~# e2fsck -f /dev/sdb2
pc ~# resize2fs /dev/sdb2

pc ~# parted /dev/sdb mkpart primary 3050,3MB 100%   # create third partition

pc ~# mkdir /mnt/sdb{1,2}
pc ~# mount /dev/sdb1 /mnt/sdb1
pc ~# mount /dev/sdb2 /mnt/sdb2

Correct to root=/dev/mmcblk0p2 in /mnt/sdb1/cmdline.txt:

dwc_otg.lpm_enable=0 console=serial0,115200 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait quiet init=/usr/lib/raspi-config/init_resize.sh

Correct /mnt/sdb2/etc/fstab to:

proc            /proc           proc    defaults          0       0
/dev/mmcblk0p1  /boot           vfat    defaults          0       2
/dev/mmcblk0p2  /               ext4    defaults,noatime  0       1

Clean up:

pc ~# umount /mnt/sdb{1,2}
pc ~# exit
pc ~$

Put the SD card into your raspi and boot. It hangs on first attempt, so I had to pull the power cord after a while with no activity on the green led. I don't know why. Second try boots.


references:
[1] Is it possible to use partition UUID for root-parameter in cmdline.txt?

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