Luke, use the force. Or in this case, Steam Deck, use the Linux. Linux has utilities like GParted, so you can partition a Drive (Hard Disk or Solid State). The hand held PC lets you replace the included SSD with a larger one, and with the recovery image, you can re-install the Steam OS. So rip out the small SSD, replace it with something larger. Re-install the Steam OS, use GPart to resize partitions if needed, and create a new one for Windoze 10. In the BIOS when you start up the deck just pick the Steam OS to boot to then install a boot manager. What else do you have to worry about, all of the Windoze 10 drivers - except audio - have been release.