I would guess that the manufacturer did something to have it be recognized as a separate drive over just another partition. I can tell you that with my experience using GParted and multiple partitions on every drive that each drive is lettered sba, sbb, sbc and so on. Each partition is sda1, sba2 and so on. So when I look at my drives in such a thing as GParted, i see sba1, sba2 sba3, sba4, sbb1, sbb2 and so on.
Like I said, I am not an expert on Linux. But from my experience this is what I have noticed. Maybe I am wrong and there is a method to trick such tools as GParted into thinking that it is a separate drive not a partition, but I have not found any such method using any software.