A factory reset, also known as a hard reset or master reset, is an effective, last resort method of troubleshooting for mobile phones. It will restore your phone to its original factory settings, erasing all your data in the process. Because of this, it's important to back up information before you do a factory reset. To do a factory reset on Galaxy S9 and Galaxy Note8, go to Settings > General Management > Reset > Factory data reset. Then tap Reset and Delete all.
You can also use hardware keys to perform a factory reset. Turn off your phone, then press and hold the Bixby key and Volume Up key, then press and hold the Power key. Release the keys when the Android mascot appears. When the Android system recovery menu appears, use the Volume Down key to select "Wipe Data/Factory Reset" and press the Power key to proceed. Select Yes and proceed again. When the factory reset completes, select "Reboot System Now".
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*Data not recoverable after factory reset.