If your Pixelbook has never been booted, it is best to boot it normally to check for any critical updates, as follows:
WARNING: This will erase any state stored locally on your Pixelbook
sudo crossystem dev_boot_usb=1
sudo crossystem dev_default_boot=usb.
The USB drive is only needed for booting when you want to re-pave or otherwise netboot the device. If you didn‘t make USB booting the default (Step #6), you will need to press Ctrl+U at the grey ‘warning OS-not verified’ screen to boot from USB when you power on your device. If the device tries to boot from USB, either because that is the default or you pressed Ctrl+U, and the device fails to boot from USB you’ll hear a fairly loud <BEEP>. Note that ChromeOS bootloader USB enumeration during boot has been observed to be slow. If you're having trouble booting from USB, it may be helpful to remove other USB devices until the device is through the bootloader and also avoid using a USB hub.
By default the ChromeOS bootloader has a long timeout to allow you to press buttons. To shortcut this you can press Ctrl+D or Ctrl+U when on the grey screen that warns that the OS will not be verified. Ctrl+D will cause the device to skip the timeout and boot from its default source. Ctrl+U will skip the timeout and boot the device from USB.
To go back to ChromeOS you must modify the priority of the Fuchsia kernel partition to be lower than that of at least one of the two ChromeOS kernel partitions.
Press Alt+Esc to get to a virtual console
Find the disk that contains the KERN-A, KERN-B, and KERN-C partitions with the
lsblk command. Below this is device 000, note how the device path of the kernel partitions is an extension of that device.
$ lsblk ID SIZE TYPE LABEL FLAGS DEVICE 000 232G /dev/sys/pci/00:1e.4/pci-sdhci/sdhci/sdmmc/block 001 5G data STATE /dev/sys/pci/00:1e.4/pci-sdhci/sdhci/sdmmc/block/part-000/block 002 16M cros kernel KERN-A /dev/sys/pci/00:1e.4/pci-sdhci/sdhci/sdmmc/block/part-001/block 003 4G cros rootfs ROOT-A /dev/sys/pci/00:1e.4/pci-sdhci/sdhci/sdmmc/block/part-002/block 004 16M cros kernel KERN-B /dev/sys/pci/00:1e.4/pci-sdhci/sdhci/sdmmc/block/part-003/block 005 4G cros rootfs ROOT-B /dev/sys/pci/00:1e.4/pci-sdhci/sdhci/sdmmc/block/part-004/block 006 64M cros kernel KERN-C /dev/sys/pci/00:1e.4/pci-sdhci/sdhci/sdmmc/block/part-005/block 007 4G cros rootfs ROOT-C /dev/sys/pci/00:1e.4/pci-sdhci/sdhci/sdmmc/block/part-006/block
gpt command to look at the device's (eg. 000) partition map.
$ gpt dump /dev/class/block/000 blocksize=0x200 blocks=488554496 Partition table is valid GPT contains usable blocks from 34 to 488554462 (inclusive) Paritition 0: STATE Start: 478035968, End: 488521727 (10485760 blocks) id: 51E8D442-0419-2447-96E5-49CB60CF0B25 type: EBD0A0A2-B9E5-4433-87C0-68B6B72699C7 flags: 0x0000000000000000 Paritition 1: KERN-A Start: 20480, End: 53247 (32768 blocks) id: 054CD627-F23C-5C40-8035-C188FA57DE9C type: FE3A2A5D-4F32-41A7-B725-ACCC3285A309 flags: priority=2 tries=0 successful=1 Paritition 2: ROOT-A Start: 8704000, End: 17092607 (8388608 blocks) id: 936E138F-1ACF-E242-9C5B-3667FAA3C10C type: 3CB8E202-3B7E-47DD-8A3C-7FF2A13CFCEC flags: 0x0000000000000000 Paritition 3: KERN-B Start: 53248, End: 86015 (32768 blocks) id: A8667891-8209-8648-9D5E-63DC9B8D0CB3 type: FE3A2A5D-4F32-41A7-B725-ACCC3285A309 flags: priority=1 tries=0 successful=1 Paritition 4: ROOT-B Start: 315392, End: 8703999 (8388608 blocks) id: 8B5D7BB4-590B-E445-B596-1E7AA1BB501F type: 3CB8E202-3B7E-47DD-8A3C-7FF2A13CFCEC flags: 0x0000000000000000 Paritition 5: KERN-C Start: 17092608, End: 17223679 (131072 blocks) id: C7D6B203-C18F-BC4D-9160-A09BA8970CE1 type: FE3A2A5D-4F32-41A7-B725-ACCC3285A309 flags: priority=3 tries=15 successful=1 Paritition 6: ROOT-C Start: 17223680, End: 25612287 (8388608 blocks) id: 769444A7-6E13-D74D-B583-C3A9CF0DE307 type: 3CB8E202-3B7E-47DD-8A3C-7FF2A13CFCEC flags: 0x0000000000000000
KERN-C typically hosts the Zircon kernel. KERN-A and KERN-B typically have ChromeOS kernels. To go to ChromeOS we need to lower the priority of KERN-C here by referencing the partition index on the disk that has that partition.
$ gpt edit_cros 5 -P 0 /dev/class/block/000
To go back to the Fuchsia kernel, just re-pave the device.