diff --git a/writer/FileWriter.cpp b/writer/FileWriter.cpp
index a566cc01..6e55f9b5 100644
--- a/writer/FileWriter.cpp
+++ b/writer/FileWriter.cpp
@@ -222,37 +222,24 @@ void FileWriter::CheckOutputFilesAvailable(const StartMessage &msg, FileWriterFo
     if (msg.overwrite.value_or(false))
         return;
 
-    // Checked by the single writer that owns the master file (write_master_file - index 0 in a
-    // multi-writer TCP/ZMQ setup), and it checks the whole run, its siblings' data files included.
-    // Data files are staggered across writers by file number, so letting each writer check its own
-    // would race the siblings already creating them; one writer can speak for all of them because
-    // the master links the data files by relative name, so they share a directory by construction.
+    // Only the master file is checked, and only by the single writer that owns it (write_master_file
+    // - index 0 in a multi-writer TCP/ZMQ setup). Enumerating the data files instead would make one
+    // writer stat every file of the run: thousands of lookups for a long run with a small
+    // images_per_file, which on a network filesystem overruns the pre-flight's ACK budget and refuses
+    // an output that was perfectly writable. Those conflicts are caught per-writer at finalize.
     if (!msg.write_master_file.value_or(false))
         return;
 
-    auto refuse = [](const std::string &name) {
-        if (std::filesystem::exists(name))
-            throw JFJochException(JFJochExceptionCategory::FileWriteError,
-                                  "Output file already exists and overwrite is off: " + name);
-    };
-
     const bool nxmx = format == FileWriterFormat::NXmxLegacy
                       || format == FileWriterFormat::NXmxVDS
                       || format == FileWriterFormat::NXmxIntegrated;
-
-    if (nxmx)
-        refuse(HDF5Metadata::MasterFileName(msg));
-
-    // NXmxIntegrated puts the images in the master file, so there are no data files to check.
-    if (format == FileWriterFormat::NXmxIntegrated || format == FileWriterFormat::NoFile)
+    if (!nxmx)
         return;
 
-    const int64_t per_file = msg.images_per_file > 0
-                             ? msg.images_per_file
-                             : static_cast<int64_t>(default_images_per_file);
-    const int64_t total_images = static_cast<int64_t>(msg.number_of_images);
-    for (int64_t file_number = 0; file_number * per_file < total_images; file_number++)
-        refuse(HDF5Metadata::DataFileName(msg, file_number));
+    const std::string name = HDF5Metadata::MasterFileName(msg);
+    if (std::filesystem::exists(name))
+        throw JFJochException(JFJochExceptionCategory::FileWriteError,
+                              "Output file already exists and overwrite is off: " + name);
 }
 
 void FileWriter::Preflight(const StartMessage &request, bool trusted_path) {