I want to restore a TWRP backup on my Samsung Galaxy S6 (SM-G920F), to get back from a custom ROM (9.0) to Stock (7.0).

Restoring /system goes fine, but TWRP throws an error when it comes to restore /data : "extractTarFork() process ended with ERROR: 255 My question is : can I restore only /system and boot my original ROM ? If that doesn't work, how can I fix this error ? (I don't have a sdcard)

  • Don't you get an option to select which partitions to restore? And that error seems to be related to tar utility. You can try unpacking backup file manually using updated tar. – Irfan Latif Oct 24 '18 at 22:55
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    While restore, it shows checkboxes under each partition name. Like /boot, /system, /data. You can uncheck /data. But, I recommend, restore all partitions. It will restore to your device to stock ROM, and then you can perform a reset to delete user data. – Rahul Gopi Oct 25 '18 at 0:03
  • @Irfan Latif : sure, I can choose what to restore, my question was : if I only restore /system (after wiping system, data, and cache of course), will my phone be able to boot on the stock ROM ? For the error, I googled a bit and it could be related to missing space on the phone, or just because /data is mounted (and I cannot unmount it because my backup is in internal storage), that's why I was asking if restoring only system would let me boot – EinderJam Oct 25 '18 at 5:18
  • Definitely, device will boot with clean /data partition. Complete Android OS lives in /system and /boot. But without /data, you will lose your personal apps, apps data, settings etc. – Irfan Latif Oct 25 '18 at 7:49
  • If you are unable to restore /data due to space limitation and you don't have sdcard as well, then restore /system and /boot first and then delete those files from backup (make sure those are correctly restored). It will spare space on internal memory and then you can restore /data. If still there is shortage of space, you can put your backup file on a PC and extract the backup to /data partition after mounting /data through USB Mass Storage. It needs a PC with Linux or a phone with USB OTG support may also work. Ask if further help required. – Irfan Latif Oct 25 '18 at 7:57

[Can] I restore only /system and boot my original ROM ?

It depends upon what lies in the backup. If you took the backup of your stock ROM and you're referring to the same backup, then yes, restoring /system using that particular backup would restore stock ROM. But you would need to restore /boot as well since that is part of stock ROM too.

However, if you must restore a different ROM, even if it is a stock ROM, it is a good practice to do a clean flash or atleast overwrite existing /data and /cache files with the ones in the backup to avoid conflict between various versions of files/apps which can cause bootloop or battery drain or whatnot.

In your case, since you cannot restore /data from the backup, I advise you to take a complete backup of your current ROM (including userdata), then reset /data (except /data/media) and /cache, and then restore /system and /boot from stock ROM backup. In this way, you would end up with a pristine stock ROM and can always restore /data when you manage to fix that error you mentioned. This is the path of least problems.

  • Thank you so much ! I just have one more question : When I made my backup, TWRP didn't offered me the option to backup /boot (only /system,/data,cache,modem,etc...). Is /boot included in /system backup ? If not, I'll be facing a big problem... – EinderJam Oct 25 '18 at 12:04
  • It is strange that TWRP didn't offer boot to be backed up. No, it has to be backed up specifically and is not included in /system. You can get the boot image if you search the stock ROM for your device on the internet. Normally, custom kernels including that of custom ROMs for a device works with stock ROM too. In an extreme case, all that would happen is that you might end up getting stuck in a boot loop or on Android boot logo or on OEM logo, or your OS might start crashing apps or the OS for no good reason. – Firelord Oct 25 '18 at 14:47
  • To avoid that, I think I'll just flash the stock ROM with Odin. I'll lose my apps and settings, but I have some SmartSwitch backups that I'll be able to restore afterwards. Could you tell me where to look for my stock ROM file and how to flash it with Odin ? I'm a bit lost where it comes to choose between BL,AP and so on – EinderJam Oct 25 '18 at 16:12
  • Sorry, I have no idea where to look for stock files for Samsung devices as I do not own one. I believe XDA forums can help in that regard. Good luck. :-) – Firelord Oct 25 '18 at 18:31

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