问题：Android9 无法向u盘写入文件
报错：
2023-12-04 11:02:45.137 15640-15640 System.err com.example.permissiontest W java.io.FileNotFoundException: /storage/C5AF-1F0E/MyImages1/Image-2676.png (Permission denied)
2023-12-04 11:02:45.137 15640-15640 System.err com.example.permissiontest W at java.io.FileOutputStream.open0(Native Method)
2023-12-04 11:02:45.137 15640-15640 System.err com.example.permissiontest W at java.io.FileOutputStream.open(FileOutputStream.java:308)
2023-12-04 11:02:45.137 15640-15640 System.err com.example.permissiontest W at java.io.FileOutputStream.<init>(FileOutputStream.java:238)
2023-12-04 11:02:45.137 15640-15640 System.err com.example.permissiontest W at java.io.FileOutputStream.<init>(FileOutputStream.java:180)
2023-12-04 11:02:45.137 15640-15640 System.err com.example.permissiontest W at com.example.permissiontest.MainActivity.SaveImageUSB(MainActivity.java:203)
2023-12-04 11:02:45.137 15640-15640 System.err com.example.permissiontest W at com.example.permissiontest.MainActivity.onClick(MainActivity.java:122)
2023-12-04 11:02:45.137 15640-15640 System.err com.example.permissiontest W at android.view.View.performClick(View.java:6597)
2023-12-04 11:02:45.138 15640-15640 System.err com.example.permissiontest W at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:1194)
2023-12-04 11:02:45.138 15640-15640 System.err com.example.permissiontest W at android.view.View.performClickInternal(View.java:6574)
2023-12-04 11:02:45.138 15640-15640 System.err com.example.permissiontest W at android.view.View.access$3100(View.java:778)
2023-12-04 11:02:45.138 15640-15640 System.err com.example.permissiontest W at android.view.View$PerformClick.run(View.java:25885)
2023-12-04 11:02:45.138 15640-15640 System.err com.example.permissiontest W at android.os.Handler.handleCallback(Handler.java:873)
2023-12-04 11:02:45.138 15640-15640 System.err com.example.permissiontest W at android.os.Handler.dispatchMessage(Handler.java:99)
2023-12-04 11:02:45.138 15640-15640 System.err com.example.permissiontest W at android.os.Looper.loop(Looper.java:193)
2023-12-04 11:02:45.138 15640-15640 System.err com.example.permissiontest W at android.app.ActivityThread.main(ActivityThread.java:6718)
2023-12-04 11:02:45.138 15640-15640 System.err com.example.permissiontest W at java.lang.reflect.Method.invoke(Native Method)
2023-12-04 11:02:45.138 15640-15640 System.err com.example.permissiontest W at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
2023-12-04 11:02:45.138 15640-15640 System.err com.example.permissiontest W at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
解决方法：更改路径
本来u盘的路径为/storage/C5AF-1F0E
替换为 /mnt/media_rw/C5AF-1F0E 即可
if (Build.VERSION.SDK_INT>Build.VERSION_CODES.O){
root = root.replace("/storage","/mnt/media_rw/");
}