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I am wondering is there a way to completly delete all data on the phone(including system, kernel...) and reinstalling clean system, something that would be equivalent to formating hard drive and reinstalling system on PC.

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Yes and no. I'll explain:

TL;DR Simple method at end of post

Using your example... Formatting a hard drive on a desktop computer (depending on quick or full format) effectively writes 0s to all (or at least enough) memory addresses to where the drive will assume it's been wiped clean. Any data recovery specialist will claim otherwise though, since data can most always be recovered.

If your goal is complete eradication of all data on your phone (though I'd wonder why lol), then you absolutely have to have root permissions, and I'd recommend a full partition map for your device. Then just open up an ADB terminal on the device and issue the following command as root:

WARNING, DO NOT ATTEMPT THIS COMMAND UNLESS YOU KNOW ABSOLUTELY WHAT IT MEANS

dd if=/dev/null of=/dev/block/mmcblk0

This command will assuredly destroy all data on the phone and prevent all but the most technically savvy recovery specialist from being able to find any usable data.

TL;DR

But if all your looking to do is to start fresh with a clean OS and you're not concerned about data recovery then you can simply do a factory reset via settings menu (remove Google account first!), via recovery menu (remove Google account first!), or if you have/are able to have an unlocked bootloader then you can manually flash the stock firmware via fastboot.

  • A am not concerned about data recovery, i am concerned about system files. For example you rooted your phone and acidentaly ruined system files or malware embeded it self into the system. In these cases factory reset won't help. And also i readed somewhere that ROM is not complete system but just some part of system and that flashing a rom won't reinstall all system files but just overwrite files it needs to. Am i right about this? If yes, is there a way to reinstall system completly? If not, can you explain this to me? By the way, your answer was very informative. – JustHobby Apr 26 '18 at 6:55
  • System files are just exactly that, files located in the /system directory. This is why there is a difference between "system" apps and "user" apps: difference being that a "system" app (located in /system/app & /system/priv-app) will persist through a factory reset whereas "user" apps (located in /data/app) generally do not. This only applies to what I guess you can call a "standard" factory reset - initiated via Settings. But if you manually flashed the firmware then malware wouldn't persist without root access, and if thats the case you'd have much bigger concerns lol. – therealjayvi Apr 28 '18 at 7:22
  • I am assuming that malware has root access, what can you do in that case to make sure it won't persist? – JustHobby Apr 28 '18 at 9:03
  • I would advise you to root your phone, then run as root adb logcat, or on the device via terminal emulator ps -A and look for anything out of place. Though if you've never done this before the amount of data it returns will be highly confusing. Honestly I think your best option would be to flash an entirely stock firmware with fastboot, this is the most powerful wipe you can give your phone without the potential of irreversibly bricking your phone (which is what could happen if using the dd command I gave). – therealjayvi Apr 28 '18 at 19:53
  • Luckily this is just theoretical question that was bothering me for long time. And about rooting, i have samsung phone and my android is up to date so no exploits to get root and no flashing because of knox fuse. – JustHobby Apr 28 '18 at 20:50
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First, for reinstalling you'll need a ROM specifically matching your device (there's no such thing as a "generic installer" for Android, see e.g. Can I install a ROM made for a different device and Why are there not generic phone OS installers?)

Second, for a "clean start", it is possible to "wipe" all mentioned partitions – but usually not with the stock features of Android devices. You will need a custom recovery such as or , where you can explicitly select which partitions to "wipe" (including /system).

Note: As therealjayvi correctly pointed out, those "wipes" wont really destroy all data (forensic experts will still be able to recover) – but I guess you won't need that anyway: as soon as you install the "fresh Android" (i.e. flash the ROM), most of the data will be overwritten anyway. And if you're really concerned about remaining data blocks being recoverable by experts: encrypt your device before wiping. That's safer than the dd approach (in terms of not accidentally destroying things).

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