Just to be clear, my understanding of that
df output is it is taken from another computer (the one that uses
sda1 as a root partition), and not that you have attached a terabyte sized drive to the pi.
If having two partition as it seems is actually the right way
It's the normal way. In fact, I've never heard of someone running a pi using only one partition -- although it should be possible, it would add some constraints that would make it a bad idea, particularly for the GNU/Linux based Pi distributions. Having two works much better because of the nature of how the pi boots. The first, small, vfat partition is the "boot" partition and contains the necessary firmware and bootloader, and the linux kernel which they hand off to. The kernel then uses the second much larger partition as its root filesystem (since the kernel is loaded into and operates from memory, its own location in relation to the root fs is not important). For convenience, the first partition is then mounted on the
/boot directory. This mounting of the first partition is actually not necessary for normal operation, but it is the convention. Note the distinction between the physical "boot partition" and the conceptional "boot directory" through which it is accessed once the system is running.