diff --git a/crates/clawhdf5-tools/src/check.rs b/crates/clawhdf5-tools/src/check.rs
index b5dd3c5..8c56771 100644
--- a/crates/clawhdf5-tools/src/check.rs
+++ b/crates/clawhdf5-tools/src/check.rs
@@ -197,7 +197,23 @@ fn unopenable(path: &std::path::Path, out: &mut Out) -> std::io::Result<i32> {
             "no HDF5 signature at offset 0 or any power of two from 512".to_string(),
         ),
         Ok(off) => match clawhdf5_format::superblock::Superblock::parse(&data[off..], 0) {
-            Ok(_) => (off as u64, "file cannot be opened".to_string()),
+            Ok(sb) => match sb.data_end(off as u64, data.len() as u64) {
+                Err(clawhdf5_format::error::FormatError::TruncatedFile {
+                    stored_eof,
+                    actual_len,
+                }) => (
+                    0,
+                    format!(
+                        "file is truncated: the superblock's end-of-file address is \
+                         {stored_eof:#x} but the file is {actual_len:#x} bytes long"
+                    ),
+                ),
+                // Say what the library refused, not just that it did.
+                _ => match clawhdf5::File::from_bytes(data.clone()) {
+                    Err(e) => (off as u64, format!("file cannot be opened: {e}")),
+                    Ok(_) => (off as u64, "file cannot be opened".to_string()),
+                },
+            },
             Err(e) => (off as u64, format!("superblock: {e}")),
         },
     };
diff --git a/crates/clawhdf5-tools/tests/h5rs_interop.rs b/crates/clawhdf5-tools/tests/h5rs_interop.rs
index 1669beb..7d97cc5 100644
--- a/crates/clawhdf5-tools/tests/h5rs_interop.rs
+++ b/crates/clawhdf5-tools/tests/h5rs_interop.rs
@@ -741,8 +741,11 @@ fn check_flags_a_misaligned_chunk() {
     let o = h5rs(&["check", &bad.to_string_lossy()]);
     let s = stdout(&o);
     assert_eq!(code(&o), 1, "{s}");
+    // The library's chunk-index reader refuses the key first (as libhdf5
+    // does); either way the problem is reported against the dataset.
     assert!(
-        s.contains("/grp/gz: chunk at [103, 0] offset 103 in dimension 0 is not a multiple of the chunk size 100"),
+        s.contains("/grp/gz: chunk at [103, 0] offset 103 in dimension 0 is not a multiple of the chunk size 100")
+            || s.contains("/grp/gz: chunk index (v1 B-tree): chunked read error: bad coordinate offset [103, 0] for chunk dimensions [100, 4]"),
         "{s}"
     );
 }