You should use reader.GetBytes(0) instead.
Store the bytes in a bytes array and then save it using the BinaryFile library.
Something like:
B4X:
Sub Globals
Dim buffer(0) As byte
End Sub
buffer() = reader.GetBytes(0)
FileOpen(c1, "out.dat", cRandom)
BinaryFile.New1(c1, False)
BinaryFile.WriteBytes(buffer())
FileClose(c1)