diff --git a/README.md b/README.md
index db71eae..e670a49 100644
--- a/README.md
+++ b/README.md
@@ -423,7 +423,13 @@ the migrations for N1MM's own admin databases and the program it speaks with,
 and neither has anything to do with a log.
 
 On a system that does not say where Documents is — most Linux systems, until
-someone sets `XDG_DOCUMENTS_DIR` — it is `~/Documents/N1MM Logger+`.
+someone sets `XDG_DOCUMENTS_DIR` — it is `~/Documents/N1MM Logger+`. Setting
+`NONEMM_HOME` puts the whole lot somewhere else, which is what a station keeping
+its files off the Documents folder, or running two copies side by side, wants.
+
+If a folder cannot be made — something else is already sitting at that path, or
+the disk will not have it — the program says so in a window and stops, rather
+than dying with a stack trace before it has a window to say anything in.
 
 ### Operators
 
diff --git a/src/Nonemm.App/App.axaml.cs b/src/Nonemm.App/App.axaml.cs
index d1c5a40..88d35d3 100644
--- a/src/Nonemm.App/App.axaml.cs
+++ b/src/Nonemm.App/App.axaml.cs
@@ -17,7 +17,12 @@ public partial class App : Application
         if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
         {
             UserPaths paths = UserPaths.Default();
-            paths.CreateFolders();
+            if (paths.CreateFolders() is { } problem)
+            {
+                desktop.MainWindow = new StartupProblemWindow(problem, paths.Root);
+                base.OnFrameworkInitializationCompleted();
+                return;
+            }
             Settings settings = Settings.Load(paths.SettingsFile);
             Themes.Use(settings.Theme);
             Themes.Changed += (_, _) => ApplyTheme();
diff --git a/src/Nonemm.App/Configuration/UserPaths.cs b/src/Nonemm.App/Configuration/UserPaths.cs
index 11c85d3..c9b1a70 100644
--- a/src/Nonemm.App/Configuration/UserPaths.cs
+++ b/src/Nonemm.App/Configuration/UserPaths.cs
@@ -33,7 +33,13 @@ public sealed class UserPaths
         Own = Path.Combine(root, "Nonemm");
     }
 
-    public static UserPaths Default() => new(DefaultRoot());
+    /// `NONEMM_HOME` moves the whole lot somewhere else, for a station that
+    /// keeps its files off the Documents folder or runs two copies side by
+    /// side.
+    public static UserPaths Default() =>
+        new(Environment.GetEnvironmentVariable("NONEMM_HOME") is { Length: > 0 } wanted
+            ? wanted
+            : DefaultRoot());
 
     public string Root { get; }
 
@@ -97,12 +103,34 @@ public sealed class UserPaths
             ? Wav
             : Path.Combine(Wav, operatorCallsign.Trim().ToUpperInvariant());
 
-    public void CreateFolders()
+    /// Makes every folder, and answers with what stopped it rather than
+    /// throwing: this runs before there is a window to report anything in, and
+    /// a program that dies with a stack trace tells the operator nothing.
+    public string? CreateFolders()
     {
         foreach (string folder in All())
         {
-            Directory.CreateDirectory(folder);
+            if (Directory.Exists(folder))
+            {
+                continue;
+            }
+            if (File.Exists(folder))
+            {
+                return $"{folder} is a file, not a folder. Move it out of the way and start again.";
+            }
+            try
+            {
+                Directory.CreateDirectory(folder);
+            }
+            catch (Exception failure) when (failure is IOException or UnauthorizedAccessException)
+            {
+                // something is already there that is neither a file nor a
+                // folder — a link to somewhere that is not there is the usual
+                // one — or the disk will not have it
+                return $"{folder} could not be made: {failure.Message}";
+            }
         }
+        return null;
     }
 
     /// Every folder that is made at startup, the root first.
diff --git a/src/Nonemm.App/Windows/StartupProblemWindow.cs b/src/Nonemm.App/Windows/StartupProblemWindow.cs
new file mode 100644
index 0000000..55d9368
--- /dev/null
+++ b/src/Nonemm.App/Windows/StartupProblemWindow.cs
@@ -0,0 +1,52 @@
+using Avalonia.Controls;
+using Avalonia.Layout;
+using Avalonia.Media;
+
+namespace Nonemm.App.Windows;
+
+/// What the operator sees when the program cannot make the folders it keeps
+/// everything in. There is no log, no settings and no entry window at that
+/// point, so this says what is wrong and what to do about it, and closing it
+/// ends the program.
+public sealed class StartupProblemWindow : Window
+{
+    public StartupProblemWindow(string problem, string root)
+    {
+        Title = "Nonemm cannot start";
+        Width = 560;
+        SizeToContent = SizeToContent.Height;
+        WindowStartupLocation = WindowStartupLocation.CenterScreen;
+        Content = new StackPanel
+        {
+            Margin = new Avalonia.Thickness(18),
+            Spacing = 10,
+            Children =
+            {
+                new TextBlock
+                {
+                    Text = "The program keeps its files with N1MM's, and could not make them:",
+                    TextWrapping = TextWrapping.Wrap,
+                },
+                new TextBlock { Text = problem, TextWrapping = TextWrapping.Wrap, FontWeight = FontWeight.Bold },
+                new TextBlock
+                {
+                    Text = $"The folder it works in is {root}. Setting NONEMM_HOME to another "
+                        + "path moves the whole lot somewhere else.",
+                    TextWrapping = TextWrapping.Wrap,
+                    FontSize = 11,
+                    Opacity = 0.75,
+                },
+                new Button
+                {
+                    Content = "Quit",
+                    HorizontalAlignment = HorizontalAlignment.Right,
+                    IsDefault = true,
+                },
+            },
+        };
+        if (((StackPanel)Content).Children[^1] is Button quit)
+        {
+            quit.Click += (_, _) => Close();
+        }
+    }
+}
diff --git a/tests/Nonemm.App.Tests/UserPathsTests.cs b/tests/Nonemm.App.Tests/UserPathsTests.cs
index 5b6a800..f623bf5 100644
--- a/tests/Nonemm.App.Tests/UserPathsTests.cs
+++ b/tests/Nonemm.App.Tests/UserPathsTests.cs
@@ -20,7 +20,7 @@ public class UserPathsTests : IDisposable
     {
         UserPaths paths = new(root);
 
-        paths.CreateFolders();
+        Assert.Null(paths.CreateFolders());
 
         string[] made = [.. Directory.GetDirectories(root).Select(Path.GetFileName).OfType<string>().Order()];
         Assert.Equal(
@@ -62,4 +62,32 @@ public class UserPathsTests : IDisposable
 
         Assert.Equal(Path.Combine(root, "Nonemm", "settings.json"), paths.SettingsFile);
     }
+
+    /// A file where the folder should be stops the program starting, so it says
+    /// so rather than throwing at an operator who has no window to read it in.
+    [Fact]
+    public void AFileWhereTheFolderShouldBeIsReported()
+    {
+        Directory.CreateDirectory(root);
+        string inTheWay = Path.Combine(root, "N1MM Logger+");
+        File.WriteAllText(inTheWay, "not a folder");
+        UserPaths paths = new(inTheWay);
+
+        string? problem = paths.CreateFolders();
+
+        Assert.NotNull(problem);
+        Assert.Contains(inTheWay, problem);
+        Assert.Contains("is a file", problem);
+    }
+
+    [Fact]
+    public void FoldersThatAreAlreadyThereAreLeftAlone()
+    {
+        UserPaths paths = new(root);
+        paths.CreateFolders();
+        File.WriteAllText(Path.Combine(paths.Databases, "om5m.s3db"), "log");
+
+        Assert.Null(paths.CreateFolders());
+        Assert.Equal("log", File.ReadAllText(Path.Combine(paths.Databases, "om5m.s3db")));
+    }
 }