I would avoid using / and \ as file path separator at all.
Using Java8 Path API, you can write code without / and \ at all. See the following example.
import java.nio.file.Files
import java.nio.file.Path
import java.nio.file.Paths
String projectDir = RunConfiguration.getProjectDir()
Path projectPath = Paths.get(projectDir)
Path dataFilesPath = projectPath.resolve('Data Files') // you do not need / and \ here
File dataFilesDir = dataFilesPath.toFile()
// use dataFilesDir as you want
This code is OS-independent. This would work on Windows, Mac and Linux without any change.