SecurityError: fileWriteResource, How to make a file in sdcard?

It's hard to say without looking at your code but here's a snippet that works.  Creates a file at "/sdcard/MyNewFile.txt".

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var fileName:String = "MyNewFile.txt"

var saveFile:File = File.documentsDirectory;

saveFile = saveFile.resolvePath(fileName);

var fileStream:FileStream = new FileStream();

fileStream.open( saveFile, FileMode.WRITE );

fileStream.writeUTFBytes( "some text" );

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Also, make sure you don't have your SD Card mounted on your computer as "USB Mass Storage".  You won't be able to access from your app it if it's mounted.

If you still can't get it working, please post a code snippet.

-David