🛠 Boot sur SD, partitions et procédure de recouvrement

Objectives

Understand the atypical boot chain of the Model B Rev 1 and learn how to restore a non-working card. Specifics: no EEPROM and no OTP boot configuration, so no USB boot — the VideoCore ROM loads bootcode.bin directly from the FAT partition of the SD card (standard format, not microSD). Recovery consists of re-flashing the card. You will learn how to write an image, diagnose boot failures via the LEDs, and save your data before re-flashing.

Hardware

• Raspberry Pi Model B Rev 1 + standard SD card (the card's only boot/root storage)
• SD card reader on a PC, Raspberry Pi OS legacy 32-bit (Buster) image
• balenaEtcher / Raspberry Pi Imager / dd on the PC
• A second Linux machine (or another card) for data recovery.

Complete code

# 1) Ecrire l'image sur la SD depuis le PC (verifier le disque !)
sudo dd if=2021-05-07-raspios-buster-armhf.img of=/dev/sdX bs=4M status=progress conv=fsync
sync

# 2) Diagnostic d'un boot qui echoue (LED verte du Pi) :
#    eteinte ou clignotements irreguliers = carte lisible mais image invalide
#    clignote normalement puis fixe = kernel charge

# 3) Sauver ses donnees AVANT re-flash : monter la partition root
#    sur un autre Linux (la partition 2 est ext4)
sudo mount /dev/sdX2 /mnt && cp -r /mnt/home/pi /salvegarde && sudo umount /mnt

# 4) Recouvrement minimal : reecrire seulement la partition FAT de boot
#    (bootcode.bin, start.elf, config.txt) si le rootfs est sain
ls /mnt   # doit contenir bootcode.bin, start.elf, config.txt, fixup.dat

Expected verification

After re-flashing, the green LED blinks regularly at startup, then the card boots to the desktop/terminal. lsblk on the Pi shows the root partition extended on the first reboot. In case of a dead card: no green LED blinking, test another SD slot and another card.

Common errors

• Attempting a USB boot: explicitly unavailable on the original Pi, unlike recent models with EEPROM.
• Writing the compressed .img.xz image instead of the decompressed image (some tools handle this, dd does not).
• Using /dev/sdX instead of /dev/sdX1 is correct for dd, but an incorrect drive letter will wipe the PC: verify with lsblk beforehand.
• Worn-out SD card: old SD cards from Pi 1s often fail; symptoms = random corrupted files, re-flashing that fixes nothing.
• Modifying config.txt with options from recent models (extreme overclocking): the Pi then refuses to boot; remove the SD card and run it on a PC to repair the FAT partition.