Fix regression with rooted paths without volume labels on Windows

#92

src/Zio/FileSystems/PhysicalFileSystem.cs

@@ -968,6 +968,11 @@ protected override string ConvertPathToInternalImpl(UPath path)
         return absolutePath;
     }
 
+    private static bool HasWindowsVolumeLabel(string path)
+    {
+        return path.IndexOf(":\\", StringComparison.Ordinal) == 1;
+    }
+
     /// <inheritdoc />
     protected override UPath ConvertPathFromInternalImpl(string innerPath)
     {
@@ -977,9 +982,10 @@ protected override UPath ConvertPathFromInternalImpl(string innerPath)
             if (innerPath.StartsWith(@"\\", StringComparison.Ordinal) || innerPath.StartsWith(@"\?", StringComparison.Ordinal))
                 throw new NotSupportedException($"Path starting with `\\\\` or `\\?` are not supported -> `{innerPath}` ");
 
-            var absolutePath = Path.IsPathRooted(innerPath) ? innerPath : Path.GetFullPath(innerPath);
-            var driveIndex = absolutePath.IndexOf(":\\", StringComparison.Ordinal);
-            if (driveIndex != 1)
+            var absolutePath = HasWindowsVolumeLabel(innerPath) ? innerPath : Path.GetFullPath(innerPath);
+
+            // Assert that Path.GetFullPath returned the format we expect
+            if (!HasWindowsVolumeLabel(absolutePath))
                 throw new ArgumentException($"Expecting a drive for the path `{absolutePath}`");
 
             var builder = UPath.GetSharedStringBuilder();