Troubleshooting & messages · Moving, offloading and restoring apps
What does "An app started writing to this while you were deciding, so it was left alone" mean?
Short answer: An app began using the item between listing and acting, so Storage Studio left it alone. Run the action again after the app finishes or is closed.
Storage Studio checks each item when it lists it and again right before it acts. Between those two moments an app started using this one, so acting on it now could disrupt that app. It was left alone.
Once the app has finished — or after you quit it — run the action again.