Originally Posted by wmoecke
Thank you, I had no clue how, but my phone's (MUCH Snail, aka MUCH W1, aka MUCH 78P01) manufacturer (China's Snail Mobile) had removed from Android's Developer Options menu the option to Enable/Disable USB Debugging.
I was looking all over the web for days, this was the only way I could effectively turn on USB debugging in my phone. Thanks!
There's only one difference, instead of performing the above steps from an ADB console window (for obvious reasons) I did it with Build.prop Editor, but first I needed to change permissions to 666 on the default.prop file (otherwise it wouldn't save my changes).
Curiously, every time I do a full reboot the default.prop file somehow is overwritten with the manufacturer's original settings - that doesn't happen when I soft reboot my device though. Any clues as to what could be reversing my changes at boot?
Everytime you boot your phone, the root path is extracted from ramdisk which is usually bound to kernel. As a result, your modification is overwritten. In order to modify files under root path(such as default.prop), you must change the file in ramdisk.
There is many tools and tutorials about this task, I think this is a good point to google.
The poster of this thread try to edit default.prop via adb, which is not possible to achieve, at least, for every android device I has hold.
For the unauthorized issue, It's because secure adb, which could be disabled via ro.adb.secure=0, but if there is a ro.adb.secure=1 in default.prop, modification of ramdisk is needed.