I've tried researching this before, but was unable to encounter an answer. Can you boot a rooted device through USB-C, SD, or something? I'm currently running a Galaxy S9+ and want to keep Android Pie on the internal storage, but I want to choose to boot from an external device through the bootloader (if there is one). I know how to access recovery mode and such (the mode where you hold the Bixby button, volume up button, and power button), but I don't know if a bootloader can be accessed. Please inform me if it is possible to boot from external hardware via a bootloader, not sure how strict Google is.

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    Google isn't strict in this regard, Samsung is. But yes, it's not impossible to boot from external sd card. On a Qualcomm device for instance, you need to replace every instance of mmcblk0 with mmcblk1. This includes - but not limited to - modification of RAM disk (like init .rc) files inside boot.img. For that, bootloader needs to be unlocked. On Treble devices, you will also need to modify device tree blob (dtb) which is usually appended to kernel image, also inside boot.img. Kernel cmdline from boot.img and fstab file from /vendor partition also need to be modified. Commented Apr 9, 2019 at 18:01
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    This is how I did it on my old device: forum.xda-developers.com/android/help/…. But this is very device specific; you won't get any practical help unless one has exactly same device as yours and he is willing to do exactly what you want. Commented Apr 9, 2019 at 18:03

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You always have to modify boot.img, so the answer is no. If you considering to edit boot.img and manage to mount the external disk to /system, you should mount another partition to /data too.
There are apps like Multirom out there which can loop mount ext4 image files located on internal storage. Afaik not for pie, you should try with some old device first before soft bricking your S9+

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