arduino/Arduino · error · IOException

'Unable to create folder: ' + destFile

Error message

'Unable to create folder: ' + destFile

What it means

FileUtils.copy recursively copies a folder tree and throws IOException "Unable to create folder: <path>" when File.mkdir() fails for a needed subdirectory. This typically happens when the destination is not writable, a same-named file already exists, or a parent directory is missing. The message names the exact directory that could not be created.

Source

Thrown at arduino-core/src/processing/app/helpers/FileUtils.java:72

      fis = new FileInputStream(source);
      fos = new FileOutputStream(dest);
      byte[] buf = new byte[4096];
      int readBytes = -1;
      while ((readBytes = fis.read(buf, 0, buf.length)) != -1) {
        fos.write(buf, 0, readBytes);
      }
    } finally {
      IOUtils.closeQuietly(fis);
      IOUtils.closeQuietly(fos);
    }
  }

  public static void copy(File sourceFolder, File destFolder) throws IOException {
    for (File file : sourceFolder.listFiles()) {
      File destFile = new File(destFolder, file.getName());
      if (file.isDirectory() && !SOURCE_CONTROL_FOLDERS.contains(file.getName())) {
        if (!destFile.exists() && !destFile.mkdir()) {
          throw new IOException("Unable to create folder: " + destFile);
        }
        copy(file, destFile);
      } else if (!file.isDirectory()) {
        copyFile(file, destFile);
      }
    }
  }

  public static void recursiveDelete(File file) {
    if (file == null) {
      return;
    }
    if (file.isDirectory()) {
      File[] files = file.listFiles();
      if (files == null) {
        return;
      }
      for (File current : files) {

View on GitHub (pinned to a0df6e0e83)

Solutions

1. Check that the destination folder exists and is writable by the current user
2. Delete/rename any FILE that has the same name as the folder being created
3. Create parent directories first (destFolder.mkdirs()) before copying
4. Run with sufficient privileges or choose a writable destination

Example fix

// before
File destFolder = new File(sketchbook, "libraries/MyLib");
FileUtils.copy(src, destFolder);
// after
if (!destFolder.exists() && !destFolder.mkdirs()) {
  throw new IOException("Cannot create destination " + destFolder);
}
FileUtils.copy(src, destFolder);

if (!destFolder.isDirectory()) destFolder.mkdirs();
if (!destFolder.canWrite()) throw new IOException("Not writable: " + destFolder);
File probe = new File(destFolder, sourceFolder.listFiles()[0].getName());
if (probe.exists() && probe.isFile() && sourceFolder.listFiles()[0].isDirectory())
  throw new IOException("File blocks folder: " + probe);

boolean canCopyInto(File dest) {
  return dest != null && (dest.isDirectory() || dest.mkdirs()) && dest.canWrite();
}

try {
  FileUtils.copy(src, dest);
} catch (IOException e) {
  if (e.getMessage().startsWith("Unable to create folder"))
    fixPermissionsOrPath(new File(e.getMessage().replace("Unable to create folder: ", "")));
  else throw e;
}

When it happens

Trigger: Calling FileUtils.copy(sourceFolder, destFolder) when a subdirectory destFolder/<name> does not exist and destFile.mkdir() returns false (no write permission on destFolder, a file with the same name exists, or destFolder itself is missing/nonexistent).

Common situations: Copying example libraries or sketches into a read-only sketchbook; destination path where a FILE exists with the folder's name; copying into a path whose parent was deleted or not yet created; running without permissions (Program Files on Windows, root-owned dirs on Linux).

Understand the failure class

Background: mkdir permission denied (EACCES): failed to create directory errors explained — this error's family across 32 libraries.

Related errors

• Error loading {0}

AI-assisted analysis of arduino/Arduino@a0df6e0e83 (2026-09-06). Data as JSON: /api/errors/8535ce123ccdd22f. Report an issue: GitHub.