What the various guides often omit.
- Run android-sdk/tools/android and update platform-tools
- The oem unlock setting on phone kept reverting. But “fastboot oem unlock” fixed it.
- Ensure “adb devices” shows the device
- Ensure “Device State” on phone shows as UNLOCKED
- “cannot load ‘system.img'” may happen because it requires several GB of free memory. So try chunking it: android-sdk/platform-tools/fastboot flash -S 512M system system.img