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Digging for that, I found various resources on the net to boot Android from SD cards. Doing that would make possible many things that are generally unavailable on phones.

For example, it would be possible to dump the whole content of the flash or get to root without the need to reformat some partitions. Effectively, it would be like unlocking PCs by a boot pen drive.

Is it possible? Can I simply burn a firmware image into an SD card and then boot my Android phone with it?

This question already gives some insight into repairing a specific model with SD card booting, but it has little to say about SD card booting in general. It does not answer my question.

Andrew T.
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peterh
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  • A locked bootloader only executes an system/kernel image that has been signed by the manufacturer. Therefore even if your device would support boot from SD (most device AFAIK do not support this) the software still would have to be the original manufacturer image. – Robert Mar 10 '20 at 12:39
  • @Robert Thanks, it helps a lot! So, I need to switch to a new bootloader, like TWRP? – peterh Mar 10 '20 at 13:18
  • @peterh No, TWRP is a recovery ROM. The bootloader is loaded before either the recovery or the system OS is loaded. To "update" the bootloader you have to unlock it (if it is possible on your device) and this is the step that erases the device. Unlocking the bootloader is usually the first step for rooting. – Robert Mar 10 '20 at 13:39
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    Booting from SDCard cannot be answered in general. It's a device specific thing. I have been able to boot a few devices from SD card but it's not possible by simply burning a firmware image. You need to create an exact image of internal eMMC/UFS chip on SD card. SoC, bootloaders, TZ, modem etc. related partitions are still read from internal storage (because those have hard coded paths in firmware). All you can do is to modify post-kernel (init's) configuration to start booting from partitions on SD card. But from your question it's not clear what benefit you want to have by doing this. – Irfan Latif Mar 11 '20 at 03:59
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    Also some devices (at least some from Samsung, or all, not sure) allow booting from SD card for some recovery purpose. SoCs on development boards almost always allow this. So do some MTK devices. But I haven't got a chance to use any such Android device so far. I've made successful attempts only with Qualcomm devices. And technically it's not "completely booting from SD card". If something at pre-kernel stage (like a bootloader) breaks, device won't boot (unless recovered, if possible). And everything that loads before Linux kernel is signed and cannot be modified, as Robert said. – Irfan Latif Mar 11 '20 at 04:08

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