Booting NetBSD from a wedge, the hard way

I have a Raspberry Pi 3 with NetBSD 10, running CI jobs. Because SD cards are notoriously unreliable, I attached a USB hard drive to it. The HDD has a swap partition and scratch space for the builds, while root is on the SD. Unfortunately, some writes end up going to the root file system after all, which meant that the SD card was destroyed after only about a year!