What case do you have? No open internal bays? I've not seen a case (save for SFF Dell/HPs) that don't have at least one if not two spots.
Discussion
I have a Dell 3070 micro.
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.
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!