SD init now happens before Wi-Fi AP startup.

Startup timing is made slower and more forgiving:
recoveryRailOffMs = 400
recoveryRailOnSettleMs = 1200
startupWarmupMs = 2500
If the initial cold-boot mount still fails, it now does one immediate second-chance remount after a short delay before proceeding.

exercises/18_GPS_Field_QA/lib/field_qa/Config.h

@@ -84,6 +84,8 @@ static constexpr float kExcellentHdop = 1.5f;
 static constexpr size_t kBufferedSamples = 10;
 static constexpr size_t kMaxSatellites = 64;
 static constexpr size_t kStorageBufferBytes = 4096;
+static constexpr uint8_t kStorageWriteRetryCount = 3;
+static constexpr uint32_t kStorageWriteRetryDelayMs = 25;
 static constexpr uint32_t kClockDisciplineRetryMs = 5000;
 static constexpr uint32_t kClockPpsWaitTimeoutMs = 1500;
 static constexpr uint32_t kClockFreshSampleMs = 2000;

exercises/18_GPS_Field_QA/lib/field_qa/StorageManager.cpp

@@ -178,6 +178,17 @@ bool StorageManager::openFile() {
   return true;
 }
 
+bool StorageManager::reopenFileForAppend() {
+  if (m_file) {
+    m_file.close();
+  }
+  m_file = SD.open(m_path.c_str(), FILE_WRITE);
+  if (!m_file) {
+    return false;
+  }
+  return true;
+}
+
 void StorageManager::writeHeader(const char* runId, const char* bootTimestampUtc) {
   if (!m_file || !m_newFile) {
     return;
@@ -206,9 +217,7 @@ bool StorageManager::writePendingBuffer() {
     if (!m_bufferPending[i] || m_bufferLengths[i] == 0) {
       continue;
     }
-    const size_t wrote = m_file.write((const uint8_t*)m_buffers[i], m_bufferLengths[i]);
-    if (wrote != m_bufferLengths[i]) {
-      m_lastError = "SD.write failed";
+    if (!writeFully((const uint8_t*)m_buffers[i], m_bufferLengths[i], "buffer")) {
       m_ready = false;
       return false;
     }
@@ -226,9 +235,7 @@ bool StorageManager::appendBytes(const char* data, size_t len) {
     if (!writePendingBuffer()) {
       return false;
     }
-    const size_t wrote = m_file.write((const uint8_t*)data, len);
-    if (wrote != len) {
-      m_lastError = "SD.write large block failed";
+    if (!writeFully((const uint8_t*)data, len, "large")) {
       m_ready = false;
       return false;
     }
@@ -256,6 +263,57 @@ bool StorageManager::appendBytes(const char* data, size_t len) {
   return true;
 }
 
+bool StorageManager::writeFully(const uint8_t* data, size_t len, const char* context) {
+  if (!m_file || !data || len == 0) {
+    m_lastError = "writeFully invalid state";
+    return false;
+  }
+
+  size_t totalWrote = 0;
+  for (uint8_t attempt = 0; attempt <= kStorageWriteRetryCount; ++attempt) {
+    const size_t filePos = (size_t)m_file.position();
+    const size_t fileSize = (size_t)m_file.size();
+    const size_t wrote = m_file.write(data + totalWrote, len - totalWrote);
+    if (wrote > 0) {
+      totalWrote += wrote;
+      if (totalWrote >= len) {
+        if (attempt > 0) {
+          m_lastError = "";
+        }
+        return true;
+      }
+    }
+
+    if (attempt >= kStorageWriteRetryCount) {
+      const bool stillMounted = mounted();
+      char err[192];
+      snprintf(err,
+               sizeof(err),
+               "SD.write failed ctx=%s wrote=%u/%u pos=%u size=%u mounted=%s attempts=%u",
+               context ? context : "?",
+               (unsigned)totalWrote,
+               (unsigned)len,
+               (unsigned)filePos,
+               (unsigned)fileSize,
+               stillMounted ? "yes" : "no",
+               (unsigned)(attempt + 1));
+      m_lastError = err;
+      return false;
+    }
+
+    delay(kStorageWriteRetryDelayMs);
+
+    if (mounted()) {
+      if (!reopenFileForAppend()) {
+        delay(kStorageWriteRetryDelayMs);
+      }
+    }
+  }
+
+  m_lastError = "SD.write retry loop exhausted";
+  return false;
+}
+
 bool StorageManager::appendLine(const String& line) {
   if (line.endsWith("\n")) {
     return appendBytes(line.c_str(), line.length());

exercises/18_GPS_Field_QA/lib/field_qa/StorageManager.h

@@ -40,11 +40,13 @@ class StorageManager {
  private:
   bool ensureDir();
   bool openFile();
+  bool reopenFileForAppend();
   void writeHeader(const char* runId, const char* bootTimestampUtc);
   String makeFilePath(const char* runId) const;
   bool appendLine(const String& line);
   bool appendBytes(const char* data, size_t len);
   bool writePendingBuffer();
+  bool writeFully(const uint8_t* data, size_t len, const char* context);
   size_t countLogsRecursive(const char* path) const;
   void listFilesRecursive(File& dir, Stream& out);
   void eraseLogsRecursive(File& dir);