I've played with those. IIRC they have 2 internal drives -- one 2.5 and a M2 socket right. Are you already using both? If so, that does stink.
Discussion
I may have found a way with Start9. As it was my favorite node software, I tried again.
After flashing, I installed it selecting my internal drive.
Then, I removed the USB drive, and did the initial setup. And I selected my external drive to store data.
Looks like I got what I wanted!