Document ChromeOS Flex install on internal eMMC and GRUB chainload dual-boot

CLAUDE.md

@@ -15,7 +15,8 @@ This is a real **HP Chromebook 13 G1** ("chell" board), converted from ChromeOS
 - BIOS/firmware: `coreboot`, version `MrChromebox-2606.1` (release date 07/14/2026) — confirms MrChromebox custom firmware, not stock ChromeOS firmware. (Was `MrChromebox-2512.2` / 01/25/2026 as of initial provisioning 2026-09-03; updated via MrChromebox firmware utility same day — see note below.)
 - System: Manufacturer `Google`, Product Name `Chell`, Family `Google_Glados`, Serial `5CD732BLC9` — `Chell`/`Glados` is the Google Chrome OS board/reference codename for the HP Chromebook 13 G1.
 - Baseboard: Manufacturer `Google`, Product Name `Chell`, Serial `PFYTU018J90043`.
-- Implication: fanless Core m CPU, no way to add a fan — keep sustained-load expectations modest; eMMC (`mmcblk0`, ~29GB) is the original ChromeOS storage, while `/` lives on `sda` (117GB), suggesting an internal drive swap/upgrade was done as part of the conversion.
+- Implication: fanless Core m CPU, no way to add a fan — keep sustained-load expectations modest.
+- **Storage correction (2026-09-04)**: `sda` (117GB, where `/` lives) is actually an external USB/SD card (`lsblk` shows `TRAN=usb`) — the whole KDE neon install boots from that external card, not from anything internal. The real internal eMMC (`mmcblk0`, ~29GB) was blank until 2026-09-04, when ChromeOS Flex was installed onto it — see "Dual boot" section below. (Superseded the original note here, which incorrectly assumed `sda` was internal.)
 
 ## Kernel 7.x black-screen bug (diagnosed and fixed 2026-09-03)
 
@@ -86,3 +87,58 @@ so this looks like a one-off "settle in" hiccup from the fresh firmware
 flash itself, not a recurrence of the underlying i915 bug or a new
 problem. If it happens again right after a firmware update, try a forced
 power-off + power-on before assuming anything is actually broken.
+
+## Dual boot: KDE neon (USB) + ChromeOS Flex (internal eMMC)
+
+2026-09-04: installed ChromeOS Flex onto the internal eMMC
+(`/dev/mmcblk0`), which was blank (see storage correction above — the
+KDE neon system boots from an external USB/SD card on `sda`, not from
+anything internal, so writing to the eMMC was safe to do live from the
+running system with zero risk to the neon install).
+
+**How it was installed**: the Flex recovery image
+(`chromeos_16640.57.0_reven_recovery_stable-channel_RevenMPKeys-v10.bin.zip`,
+in `matt`'s `~/Downloads`) is a full raw GPT disk image — exactly what
+the normal "boot from USB, then Install CloudReady/Chrome OS Flex" flow
+`dd`s onto the target disk. Since neon's root already lives on external
+media, that install flow was skipped entirely and the image was
+streamed straight onto the eMMC instead:
+
+```
+sudo sh -c 'unzip -p "<path to .zip>" <path to .bin inside the zip> \
+  | dd of=/dev/mmcblk0 bs=4M status=progress conv=fsync'
+```
+
+Unzipped image is ~8.9GB; eMMC is 29GB, plenty of room (ChromeOS auto-
+expands the stateful partition to fill the rest on first real boot).
+eMMC write speed degraded over the transfer (~130MB/s at the start down
+to ~9MB/s by the end, total ~18 min for 8.9GB) — looks like the eMMC's
+SLC write cache exhausting partway through; expected/normal for eMMC of
+this class, not a fault.
+
+**Boot selection** — both OSes are reachable two ways:
+1. **From GRUB** (neon's own boot menu, visible for 5s per
+   `GRUB_TIMEOUT`): a custom entry "ChromeOS Flex (internal eMMC)" was
+   added to `/etc/grub.d/40_custom`, which chainloads the eMMC's own EFI
+   bootloader:
+   ```
+   menuentry "ChromeOS Flex (internal eMMC)" {
+       insmod part_gpt
+       insmod fat
+       insmod chain
+       search --no-floppy --fs-uuid --set=root CBE4-703E
+       chainloader /EFI/BOOT/BOOTX64.EFI
+   }
+   ```
+   `CBE4-703E` is the FAT filesystem UUID of the eMMC's `EFI-SYSTEM`
+   partition (`mmcblk0p12`); `/EFI/BOOT/BOOTX64.EFI` is Chrome OS's
+   fallback UEFI loader (`crdyboot`) on that partition. Run
+   `sudo update-grub` after editing `40_custom` for changes to take
+   effect.
+2. **From the firmware's own UEFI boot picker** (MrChromebox UEFI,
+   typically `Esc` during POST) — `efibootmgr -v` shows both `neon`
+   (Boot0005, on the USB stick) and `Internal eMMC` (Boot0001) as
+   registered entries, `BootOrder` currently `0005,0001,0000,0003,0002`
+   (neon first). If the USB stick is unplugged, firmware falls through
+   past the missing `neon` entry straight to `Internal eMMC` and boots
+   Flex automatically — no menu needed.