cdm/src/main/java/ucar/nc2/util/DiskCache.java
static public void makeRootDirectory() {
    File dir = new File(root);
    if (!dir.exists())
      if (!dir.mkdirs())
        throw new IllegalStateException("DiskCache.setRootDirectory(): could not create root directory <" + root + ">.");
    checkExist = true;
  }
static public void makeRootDirectory() {
    File dir = new File(root);
    if (!dir.exists())
      if (!dir.mkdirs())
        throw new IllegalStateException("DiskCache.setRootDirectory(): could not create root directory <" + root + ">.");
    checkExist = true;
  }
Make sure that the current root directory exists.