fix: CreateSubdirectory failing for root directories

src/libraries/System.Private.CoreLib/src/System/IO/DirectoryInfo.cs

@@ -85,6 +85,12 @@ public DirectoryInfo CreateSubdirectory(string path)
             ReadOnlySpan<char> trimmedNewPath = Path.TrimEndingDirectorySeparator(newPath.AsSpan());
             ReadOnlySpan<char> trimmedCurrentPath = Path.TrimEndingDirectorySeparator(FullPath.AsSpan());
 
+            // If trimmedCurrentPath still has a trailing separator (root directory case like "C:\" or "/"),
+            // remove it for proper boundary checking
+            if (trimmedCurrentPath.Length > 0 && PathInternal.IsDirectorySeparator(trimmedCurrentPath[trimmedCurrentPath.Length - 1]))
+            {
+                trimmedCurrentPath = trimmedCurrentPath.Slice(0, trimmedCurrentPath.Length - 1);
+            }
             // We want to make sure the requested directory is actually under the subdirectory.
             if (trimmedNewPath.StartsWith(trimmedCurrentPath, PathInternal.StringComparison)
                 // Allow the exact same path, but prevent allowing "..\FooBar" through when the directory is "Foo"

src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/EnumerableTests.cs

@@ -55,6 +55,75 @@
             }
         }
 
+        [Fact]
+        [PlatformSpecific(TestPlatforms.Windows)]
+        public void CreateSubdirectory_RootDriveSubfolder_Windows()
+        {
+            // Get the root of the OS drive (e.g., "C:\")
+            string rootDrive = Path.GetPathRoot(Environment.SystemDirectory);
+
+            // Create a DirectoryInfo for the root drive
+            DirectoryInfo rootDirectory = new DirectoryInfo(rootDrive);
+
+            // Create a unique test folder name to avoid conflicts
+            string testFolderName = $"TestFolder_{Guid.NewGuid():N}";
+
+            try
+            {
+                // Create a subdirectory directly in the root
+                DirectoryInfo subDirectory = rootDirectory.CreateSubdirectory(testFolderName);
+
+                // Verify it was created
+                Assert.True(subDirectory.Exists);
+                Assert.Equal(Path.Combine(rootDrive, testFolderName), subDirectory.FullName);
+            }
+            finally
+            {
+                // Cleanup
+                string testFolderPath = Path.Combine(rootDrive, testFolderName);
+                if (Directory.Exists(testFolderPath))
+                {
+                    Directory.Delete(testFolderPath, recursive: true);
+                }
+            }
+        }
+
+
+        [Fact]
+        [PlatformSpecific(TestPlatforms.Linux)]
+        public void CreateSubdirectory_RootDriveSubfolder_Linux()
+        {
+            // Get the root of the OS drive (e.g., "/")
+            // can not use string rootDrive = Path.GetPathRoot(Environment.SystemDirectory); as Environment.SystemDirectory is empty for Linux
+            string rootDrive = "/"
+
+            // Create a DirectoryInfo for the root drive
+            DirectoryInfo rootDirectory = new DirectoryInfo(rootDrive);
+
+            // Create a unique test folder name to avoid conflicts
+            string testFolderName = $"TestFolder_{Guid.NewGuid():N}";
+
+            try
+            {
+                // Create a subdirectory directly in the root
+                DirectoryInfo subDirectory = rootDirectory.CreateSubdirectory(testFolderName);
+
+                // Verify it was created
+                Assert.True(subDirectory.Exists);
+                Assert.Equal(Path.Combine(rootDrive, testFolderName), subDirectory.FullName);
+            }
+            finally
+            {
+                // Cleanup
+                string testFolderPath = Path.Combine(rootDrive, testFolderName);
+                if (Directory.Exists(testFolderPath))
+                {
+                    Directory.Delete(testFolderPath, recursive: true);
+                }
+            }
+        }
+
+
         [Fact]
         public void EnumerateDirectories_NonBreakingSpace()
         {