Tested and fixed the VSS mapping problem caused by using Path.Combine with a \?\\GLOBALROOT\ prefixed path.

Duplicati/Library/Snapshots/WindowsSnapshot.cs

@@ -75,6 +75,11 @@ namespace Duplicati.Library.Snapshots
         /// </summary>
         private static SystemIOWindows IO_WIN = new SystemIOWindows();
 
+        /// <summary>
+        /// Commonly used string element
+        /// </summary>
+        private static string SLASH = Path.DirectorySeparatorChar.ToString();
+
         /// <summary>
         /// Constructs a new backup snapshot, using all the required disks
         /// </summary>
@@ -399,21 +404,33 @@ namespace Duplicati.Library.Snapshots
             throw new InvalidOperationException();
         }
 
+        /// <summary>
+        /// Cache element to speed up returning the same paths
+        /// </summary>
+        private KeyValuePair<string, string> m_lastLookup;
+
         /// <inheritdoc />
         public override string ConvertToSnapshotPath(string localPath)
         {
+            // Hot cache, we tend to call the same conversion multiple times
+            var m = m_lastLookup;
+            if (m.Key == localPath)
+                return m.Value;
+
             if (!Path.IsPathRooted(localPath))
                 throw new InvalidOperationException();
 
             var root = AlphaFS.Path.GetPathRoot(localPath);
-
             if (!m_volumeMap.TryGetValue(root, out var volumePath))
                 throw new InvalidOperationException();
 
-            var mappedPath = IO_WIN.PathCombine(volumePath, localPath.Substring(root.Length));
-            if (localPath.EndsWith("\\", StringComparison.Ordinal))
-                return Library.Utility.Utility.AppendDirSeparator(mappedPath);
+            // Note: Do NOT use Path.Combine as it strips the UNC path prefix
+            var subPath = localPath.Replace(root, String.Empty);
+            if (!volumePath.EndsWith(SLASH, StringComparison.Ordinal) && !subPath.StartsWith(SLASH, StringComparison.Ordinal))
+                subPath = subPath.Insert(0, SLASH);
 
+            var mappedPath = subPath.Insert(0, volumePath);
+            m_lastLookup = new KeyValuePair<string, string>(localPath, mappedPath);
             return mappedPath;
         }
 
@@ -454,13 +471,13 @@ namespace Duplicati.Library.Snapshots
 				catch (Exception ex)
                 {
 					Logging.Log.WriteVerboseMessage(LOGTAG, "MappedDriveCleanupError", ex, "Failed during VSS mapped drive unmapping");
-                }
-
-				try
-				{
-					m_backup?.BackupComplete();
+                }
+
+				try
+				{
+					m_backup?.BackupComplete();
 				}
-				catch (Exception ex)
+				catch (Exception ex)
 				{
 					Logging.Log.WriteVerboseMessage(LOGTAG, "VSSTerminateError", ex, "Failed to signal VSS completion");
 				}
@@ -470,12 +487,12 @@ namespace Duplicati.Library.Snapshots
                     if (m_backup != null)
                     {
                         foreach (var g in m_volumes.Values)
-                        {
-							try
-							{
-								m_backup.DeleteSnapshot(g, false);
+                        {
+							try
+							{
+								m_backup.DeleteSnapshot(g, false);
 							}
-							catch (Exception ex)
+							catch (Exception ex)
 							{
 								Logging.Log.WriteVerboseMessage(LOGTAG, "VSSSnapShotDeleteError", ex, "Failed to close VSS snapshot");
 							}