Demontager - I have your hardware and have bad news:
I run into the same partitioning problem and is one of the reasons I never do a frugal or even boot an iso from the internal mmc device - it changes randomly.
What will make your hair gray is that it also depends on whether you do a reboot, or a total shutdown and power up. Sometimes it stays the same, and sometimes it goes haywire.
In addition, if you run from say an external usb stick, do a normal shutdown, and move the stick to another port, that seems to be a problem too randomly.
The only way to start from a known state is to actually *pull the dc jack* and reinsert. You'll see the blue light come on where the system is apparently scanning the ports and shuts off again, waiting for you to power up manually.
Even powering up manually can result in different port partitions depending on whether you do a short-press power up, or a long-press (3 seconds or more held) and let it power up that way.
What I'm saying is that this particular Wintel hockey-puck is best left under a sledge-hammer. Ask me how I know!
Sorry to be the bearer of bad news, but perhaps this knowledge will add years to your life.