Use your external hard drive formatted to Btrfs for your Raspberry Pi.

Select HDD

1. Find your external HDD.
   lsblk | grep -i "sd"
2. Find the partition on external HDD.
   sudo fdisk -l
3. Enter fdisk for formatting by selecting your external HDD.
   sudo fdisk /dev/sda

fdisk

1. Delete partition. d
2. Create partition. n
3. Select partition number. 1
4. Select the first sector instead of using the default 2048, since my external HDD has range of 34-976773134, I choose 34. 34
5. Select the last sector. The default is the last sector, so just enter it.
6. Write changes to disk. This will delete all data on the external HDD. w

Btrfs format

1. Format external HDD to Btrfs with label Raspberry-Pi-HDD, label is there for easier mounting.
   sudo mkfs.btrfs -L Raspberry-Pi-HDD /dev/sda1

You can change label at any time by using this command:
sudo btrfs filesystem label /dev/sda1 Raspberry-Pi-HDD

Mounting

1. Create a mount point on /mnt folder.
   sudo mkdir /mnt/HDD
2. Mount external HDD.
   sudo mount /dev/disk/by-label/Raspberry-Pi-HDD /mnt/HDD
3. Own the external HDD. By default, it is owned by root.
   sudo chown -R $USER:$USER /mnt/HDD

Now every time you reboot your Raspberry Pi, you can mount it again using:
sudo mount /dev/disk/by-label/Raspberry-Pi-HDD /mnt/HDD

fstab

For automatic mount upon reboot, add this line on your /etc/fstab by using sudo nano /etc/fstab. Be careful with this file as it can easily cause your system not to boot.

1

LABEL=Raspberry-Pi-HDD /mnt/HDD btrfs nofail,x-systemd.device-timeout=5s 0 0

Nofail so that system will not error if the external HDD is disconnected upon boot, and systemd.device-timeout so that system will not wait 90 seconds for the device to appear if disconnected, only 5 seconds.