diff --git a/crates/ts_goport/src/execute/build/orchestrator.rs b/crates/ts_goport/src/execute/build/orchestrator.rs
index 251209caf..3913bd98e 100644
--- a/crates/ts_goport/src/execute/build/orchestrator.rs
+++ b/crates/ts_goport/src/execute/build/orchestrator.rs
@@ -214,8 +214,16 @@ pub struct Orchestrator {
 
 impl Orchestrator {
     // Go: build/orchestrator.go:94 (*Orchestrator).relativeFileName
+    // PORT: Go relativePath (:98) is inlined; the name is a file name.
     pub fn relative_file_name(&self, file_name: &str) -> String {
-        convert_to_relative_path(file_name, &self.compare_paths_options)
+        if let Some(relative) = relative_path_from_directory(
+            &self.compare_paths_options.current_directory,
+            file_name,
+            self.compare_paths_options.use_case_sensitive_file_names,
+        ) {
+            return relative;
+        }
+        file_name.to_string()
     }
 
     // Go: build/orchestrator.go:97 (*Orchestrator).toPath (at 673a5f17d713; removed by
diff --git a/crates/ts_goport/src/execute/tsc/diagnostics.rs b/crates/ts_goport/src/execute/tsc/diagnostics.rs
index 16c743831..391908f86 100644
--- a/crates/ts_goport/src/execute/tsc/diagnostics.rs
+++ b/crates/ts_goport/src/execute/tsc/diagnostics.rs
@@ -21,9 +21,23 @@ use super::compile::{System, Writer, write_str};
 use super::compile::CommandLineTesting;
 use crate::diagnostics::Category;
 use crate::diagnostics_loc::message_localize;
-use crate::frontend::tspath::{ComparePathsOptions, convert_to_relative_path, path_is_absolute};
+use crate::frontend::tspath::{ComparePathsOptions, relative_path_from_directory};
 use crate::locale::Locale;
 
+// Go: diagnosticwriter/diagnosticwriter.go:27 formatFileNameRelativeTo
+// PORT: the directory and the case sensitivity are the fields of the
+// `compare_paths_options` of `FormattingOptions`.
+pub fn format_file_name_relative_to(file_name: &str, options: &ComparePathsOptions) -> String {
+    if let Some(relative_path) = relative_path_from_directory(
+        &options.current_directory,
+        file_name,
+        options.use_case_sensitive_file_names,
+    ) {
+        return relative_path;
+    }
+    file_name.to_string()
+}
+
 // Go: diagnosticwriter/diagnosticwriter.go:203 FormattingOptions
 #[derive(Clone, Debug, Default)]
 pub struct FormattingOptions {
@@ -1051,7 +1065,7 @@ pub fn write_location(
     let (first_line, first_char) = get_ecma_line_and_utf16_character_of_file_position(&file, pos);
     let relative_file_name = match format_opts {
         Some(format_opts) => {
-            convert_to_relative_path(file.file_name(), &format_opts.compare_paths_options)
+            format_file_name_relative_to(file.file_name(), &format_opts.compare_paths_options)
         }
         None => file.file_name().to_string(),
     };
@@ -1251,12 +1265,8 @@ fn pretty_path_for_file_error(
         return String::new();
     }
     let line = get_ecma_line_of_file_position(file, AstDiagnostic(file_errors[0]).pos());
-    let mut file_name = file.file_name().to_string();
-    if path_is_absolute(&file_name)
-        && path_is_absolute(&format_opts.compare_paths_options.current_directory)
-    {
-        file_name = convert_to_relative_path(file.file_name(), &format_opts.compare_paths_options);
-    }
+    let file_name =
+        format_file_name_relative_to(file.file_name(), &format_opts.compare_paths_options);
     format!(
         "{}{}:{}{}",
         file_name,
@@ -1278,7 +1288,7 @@ pub fn write_format_diagnostic(
             get_ecma_line_and_utf16_character_of_file_position(&file, wrapped.pos());
         let file_name = file.file_name();
         let relative_file_name =
-            convert_to_relative_path(file_name, &format_opts.compare_paths_options);
+            format_file_name_relative_to(file_name, &format_opts.compare_paths_options);
         write_str(
             output,
             &format!("{}({},{}): ", relative_file_name, line + 1, character + 1),
diff --git a/crates/ts_goport/src/frontend/compiler/file_include.rs b/crates/ts_goport/src/frontend/compiler/file_include.rs
index d97c33876..756dead08 100644
--- a/crates/ts_goport/src/frontend/compiler/file_include.rs
+++ b/crates/ts_goport/src/frontend/compiler/file_include.rs
@@ -5,32 +5,23 @@ use crate::frontend::prelude::*;
 
 /// Go `caseSensitivity.RelativePathFromDirectory(directory, fileName)` and
 /// its fallback to the absolute name (ts#64159: fileInclude.go:155,
-/// program.go:2145). Used for the include-reason text and `explain_files`.
-// PORT: the names are normalized, so not reducing "." and ".." (rule R3)
-// changes nothing here. Another root, or an empty `directory` (Go tests pass
-// ""), gives the absolute name (rule R4).
+/// program.go:2159). Used for the include-reason text and `explain_files`.
+// PORT: another root, an empty `directory` (Go tests pass "") or a relative
+// name that looks rooted (`c:/x` under a directory named `c:`) gives the
+// absolute name (rule R4).
 pub(crate) fn relative_file_name_from_directory(
     directory: &str,
     file_name: &str,
     program: &NewProgram,
 ) -> String {
-    let directory_root = &directory[..get_root_length(directory)];
-    let file_root = &file_name[..get_root_length(file_name)];
-    if directory_root.is_empty()
-        || !directory_root
-            .trim_end_matches('/')
-            .eq_ignore_ascii_case(file_root.trim_end_matches('/'))
-    {
-        return file_name.to_string();
-    }
-    get_relative_path_from_directory(
+    if let Some(relative_path) = relative_path_from_directory(
         directory,
         file_name,
-        &ComparePathsOptions {
-            use_case_sensitive_file_names: program.use_case_sensitive_file_names(),
-            current_directory: directory.to_string(),
-        },
-    )
+        program.use_case_sensitive_file_names(),
+    ) {
+        return relative_path;
+    }
+    file_name.to_string()
 }
 
 // Go: fileInclude.go:14 fileIncludeKind
diff --git a/crates/ts_goport/src/program.rs b/crates/ts_goport/src/program.rs
index fdf778c56..0de419862 100644
--- a/crates/ts_goport/src/program.rs
+++ b/crates/ts_goport/src/program.rs
@@ -6518,7 +6518,10 @@ pub fn format_diagnostic(diagnostic: &Diagnostic) -> String {
         };
         output.push_str(&format!(
             "{}({},{}): ",
-            tspath::convert_to_relative_path(file_name, &compare_options),
+            crate::execute::tsc::diagnostics::format_file_name_relative_to(
+                file_name,
+                &compare_options
+            ),
             line + 1,
             character + 1
         ));
diff --git a/crates/ts_goport/tests/go_baselines/tsctests/drive_like_segment_path.rs b/crates/ts_goport/tests/go_baselines/tsctests/drive_like_segment_path.rs
new file mode 100644
index 000000000..7087ee793
--- /dev/null
+++ b/crates/ts_goport/tests/go_baselines/tsctests/drive_like_segment_path.rs
@@ -0,0 +1,225 @@
+//! Port-only test of the names of files under a directory named like a
+//! drive (`c:`).
+//!
+//! PORT: no Go counterpart. Go (ts#64159) writes the name of a file relative
+//! to the current directory with RelativePathFromDirectory
+//! (tspath/relative_path.go:82), which gives the absolute name when the
+//! relative name looks rooted: `<cwd>/c:/a.ts` is `c:/a.ts` relative, which
+//! has a DOS root. The port's diagnostic writer (diagnosticwriter.go:27
+//! formatFileNameRelativeTo), `--explainFiles` (program.go:2159,
+//! fileInclude.go:155) and `tsc -b` (build/orchestrator.go:94) printed
+//! `c:/a.ts`. A file reached through module resolution gets the real path
+//! (`<cwd>/c:/...`, osvfs os.go:142), which is how the fuzzer found it. Only
+//! a one-letter name followed by `:` is a DOS root (`x:y`, `ab:c` are not).
+//!
+//! The test runs `tsgo` in a scratch dir, as the names come from the OS
+//! realpath. Every expected output is the output of `tsgo-oracle-3e9236361fa9`
+//! on the same files, with its scratch dir replaced.
+
+use std::path::PathBuf;
+use std::process::{Command, Stdio};
+
+/// A scratch dir with the files of one case. It is deleted on drop.
+struct Project(PathBuf);
+
+impl Project {
+    fn new(name: &str, files: &[(&str, &str)]) -> Project {
+        let dir = std::env::temp_dir().join(format!(
+            "tsgo_drive_like_segment_{}_{name}",
+            std::process::id()
+        ));
+        let _ = std::fs::remove_dir_all(&dir);
+        std::fs::create_dir_all(&dir).unwrap();
+        // The names have the real path of the dir, as Go's realpath.
+        let dir = std::fs::canonicalize(&dir).unwrap();
+        for (file, text) in files {
+            let path = dir.join(file);
+            std::fs::create_dir_all(path.parent().unwrap()).unwrap();
+            std::fs::write(path, text).unwrap();
+        }
+        Project(dir)
+    }
+
+    fn dir(&self) -> String {
+        self.0.to_str().unwrap().to_string()
+    }
+
+    /// Runs `tsgo` with `args` and gives the exit code and the output.
+    fn tsc(&self, args: &[&str]) -> (Option<i32>, String) {
+        let output = Command::new(env!("CARGO_BIN_EXE_tsgo"))
+            .args(args)
+            .current_dir(&self.0)
+            .stdin(Stdio::null())
+            .output()
+            .expect("run tsgo");
+        let mut text = String::from_utf8_lossy(&output.stdout).into_owned();
+        text.push_str(&String::from_utf8_lossy(&output.stderr));
+        (output.status.code(), text)
+    }
+}
+
+impl Drop for Project {
+    fn drop(&mut self) {
+        let _ = std::fs::remove_dir_all(&self.0);
+    }
+}
+
+/// `text` without its ANSI escapes.
+fn strip_escapes(text: &str) -> String {
+    let mut out = String::new();
+    let mut chars = text.chars();
+    while let Some(c) = chars.next() {
+        if c == '\x1b' {
+            for c in chars.by_ref() {
+                if c == 'm' {
+                    break;
+                }
+            }
+        } else {
+            out.push(c);
+        }
+    }
+    out
+}
+
+/// The fuzzer's repro: a module resolved to `<cwd>/c:/node_modules/...`.
+#[test]
+fn resolved_module_under_a_drive_like_dir_has_its_absolute_name() {
+    let project = Project::new(
+        "resolved",
+        &[
+            ("c:/root/a.ts", "import \"file4\";\n"),
+            (
+                "c:/node_modules/file4/index.d.ts",
+                "let x: number = \"s\";\n",
+            ),
+            (
+                "tsconfig.json",
+                r#"{"compilerOptions":{"noEmit":true},"files":["c:/root/a.ts"]}"#,
+            ),
+        ],
+    );
+    let dir = project.dir();
+    let file4 = format!("{dir}/c:/node_modules/file4/index.d.ts");
+    let diagnostics = format!(
+        "{file4}(1,1): error TS1046: Top-level declarations in .d.ts files must start with either a 'declare' or 'export' modifier.
+{file4}(1,5): error TS2322: Type 'string' is not assignable to type 'number'.
+{file4}(1,17): error TS1039: Initializers are not allowed in ambient contexts.
+"
+    );
+
+    let (code, output) = project.tsc(&["-p", "tsconfig.json", "--pretty", "false"]);
+    assert_eq!(output, diagnostics);
+    assert_eq!(code, Some(2));
+
+    let (code, output) =
+        project.tsc(&["-p", "tsconfig.json", "--pretty", "false", "--explainFiles"]);
+    assert!(output.starts_with(&diagnostics), "{output}");
+    let explained = format!(
+        "{file4}
+   Imported via \"file4\" from file 'c:/root/a.ts'
+c:/root/a.ts
+   Part of 'files' list in tsconfig.json
+"
+    );
+    assert!(output.ends_with(&explained), "{output}");
+    assert_eq!(code, Some(2));
+}
+
+/// A root file named by its absolute path, and names with a `:` that are
+/// not a DOS root, in the pretty output and its summary.
+#[test]
+fn pretty_names_under_a_drive_like_dir_are_absolute() {
+    let project = Project::new(
+        "pretty",
+        &[
+            ("c:/root/a.ts", "import \"./b\";\nlet y: string = 1;\n"),
+            ("c:/root/b.ts", "export {}; let z: string = 2;\n"),
+            ("x:y/c.ts", "let w: string = 3;\n"),
+            ("ab:c/e.ts", "let v: string = 4;\n"),
+        ],
+    );
+    let dir = project.dir();
+    std::fs::write(
+        project.0.join("tsconfig.json"),
+        format!(
+            r#"{{"compilerOptions":{{"noEmit":true}},"files":["{dir}/c:/root/a.ts","x:y/c.ts","ab:c/e.ts"]}}"#
+        ),
+    )
+    .unwrap();
+
+    let (code, output) = project.tsc(&["-p", "tsconfig.json", "--pretty", "true"]);
+    assert_eq!(
+        strip_escapes(&output),
+        format!(
+            "ab:c/e.ts:1:5 - error TS2322: Type 'number' is not assignable to type 'string'.
+
+1 let v: string = 4;
+      ~
+
+{dir}/c:/root/a.ts:2:5 - error TS2322: Type 'number' is not assignable to type 'string'.
+
+2 let y: string = 1;
+      ~
+
+{dir}/c:/root/b.ts:1:16 - error TS2322: Type 'number' is not assignable to type 'string'.
+
+1 export {{}}; let z: string = 2;
+                 ~
+
+x:y/c.ts:1:5 - error TS2322: Type 'number' is not assignable to type 'string'.
+
+1 let w: string = 3;
+      ~
+
+
+Found 4 errors in 4 files.
+
+Errors  Files
+     1  ab:c/e.ts:1
+     1  {dir}/c:/root/a.ts:2
+     1  {dir}/c:/root/b.ts:1
+     1  x:y/c.ts:1
+
+"
+        )
+    );
+    assert_eq!(code, Some(2));
+}
+
+/// `tsc -b --verbose` of a project under a drive-like dir.
+#[test]
+fn build_names_under_a_drive_like_dir_are_absolute() {
+    let project = Project::new(
+        "build",
+        &[
+            ("c:/p/a.ts", "export const a = 1;\nlet x: number = \"s\";\n"),
+            (
+                "c:/p/tsconfig.json",
+                r#"{"compilerOptions":{"outDir":"out"},"files":["a.ts"]}"#,
+            ),
+        ],
+    );
+    let dir = project.dir();
+    let config = format!("{dir}/c:/p");
+    let (code, output) = project.tsc(&["-b", &config, "--verbose", "--pretty", "false"]);
+    // Without the time of each status line.
+    let output = output
+        .lines()
+        .map(|line| match line.split_once(" - ") {
+            Some((time, rest)) if time.ends_with("AM") || time.ends_with("PM") => rest,
+            _ => line,
+        })
+        .collect::<Vec<_>>()
+        .join("\n");
+    assert_eq!(
+        output,
+        format!(
+            "Projects in this build: \n    * {config}/tsconfig.json\n\n\
+             Project '{config}/tsconfig.json' is out of date because output file '{config}/out/tsconfig.tsbuildinfo' does not exist\n\n\
+             Building project '{config}/tsconfig.json'...\n\n\
+             {config}/a.ts(2,5): error TS2322: Type 'string' is not assignable to type 'number'."
+        )
+    );
+    assert_eq!(code, Some(2));
+}
diff --git a/crates/ts_goport/tests/go_baselines/tsctests/mod.rs b/crates/ts_goport/tests/go_baselines/tsctests/mod.rs
index 2f3d421fb..b938b5fab 100644
--- a/crates/ts_goport/tests/go_baselines/tsctests/mod.rs
+++ b/crates/ts_goport/tests/go_baselines/tsctests/mod.rs
@@ -3,6 +3,7 @@ mod build_info_corrupt;
 mod build_prefetch_rereads;
 mod checker_common_source_directory;
 mod contentmapper_watch;
+mod drive_like_segment_path;
 mod effect_build_info;
 mod explain_files_cache;
 mod extended_counts;