diff --git a/reflash/server.go b/reflash/server.go
index cbb286a..bba9b11 100644
--- a/reflash/server.go
+++ b/reflash/server.go
@@ -578,7 +578,11 @@ func uploadStart(w http.ResponseWriter, r *http.Request) {
 	timeStart = time.Now()
 	logInfo("Starting upload at " + timeStart.Format("15:04:05"))
 	logInfo("Filename: " + state.Filename)
-	os.Create(images_folder + "/" + state.Filename)
+	f, err := os.Create(images_folder + "/" + state.Filename)
+	if err != nil {
+		log.Fatal(err)
+	}
+	state.File = f
 
 	sendResponse(w, nil)
 }
@@ -674,25 +678,26 @@ func uploadChunk(w http.ResponseWriter, r *http.Request) {
 	reqBody, _ := io.ReadAll(r.Body)
 	json.Unmarshal(reqBody, &chunk)
 
-	decoded, err := base64.StdEncoding.DecodeString(chunk.Encoded[37:len(chunk.Encoded)])
-
-	path := images_folder + "/" + state.Filename
-
 	if state.State == CANCELLED {
 		response := map[string]bool{"success": false}
 		json.NewEncoder(w).Encode(response)
 		return
 	}
 
-	f, err := os.OpenFile(path, os.O_APPEND|os.O_WRONLY, 0644)
+	decoded, err := base64.StdEncoding.DecodeString(chunk.Encoded[37:len(chunk.Encoded)])
 	if err != nil {
-		log.Fatal(err)
-	}
-	if _, err := f.Write(decoded); err != nil {
-		log.Fatal(err)
+		http.Error(w, "Failed to decode base64", http.StatusBadRequest)
+		return
 	}
-	if err := f.Close(); err != nil {
-		log.Fatal(err)
+
+	// state.File is opened once in uploadStart and closed in
+	// uploadFinish/uploadCancel - opening, writing and closing it on
+	// every chunk here was real per-chunk overhead (syscalls plus an
+	// implicit flush on close), especially costly against the
+	// FAT-formatted USB target.
+	if _, err := state.File.Write(decoded); err != nil {
+		http.Error(w, "Failed to write chunk to file", http.StatusInternalServerError)
+		return
 	}
 	state.BytesNow += len(decoded)
 	state.Progress = float64(state.BytesNow) * 100 / float64(state.BytesTotal)
@@ -702,6 +707,12 @@ func uploadChunk(w http.ResponseWriter, r *http.Request) {
 }
 
 func uploadFinish(w http.ResponseWriter, r *http.Request) {
+	if state.File != nil {
+		if err := state.File.Close(); err != nil {
+			log.Fatal(err)
+		}
+		state.File = nil
+	}
 	mountUsb(MODE_RO)
 	duration := time.Since(timeStart)
 	logInfo(fmt.Sprintf("Upload finished in %d minutes and %d seconds", int(duration.Minutes()), int(duration.Seconds())%60))
diff --git a/reflash/server_test.go b/reflash/server_test.go
index 7e6bc4d..c32f64f 100644
--- a/reflash/server_test.go
+++ b/reflash/server_test.go
@@ -1,6 +1,8 @@
 package main
 
 import (
+	"bytes"
+	"encoding/base64"
 	"encoding/json"
 	"net/http/httptest"
 	"os"
@@ -393,3 +395,65 @@ func TestGetUncompressedSizeRealXz(t *testing.T) {
 		t.Errorf("getUncompressedSize = %d, want %d", got, size)
 	}
 }
+
+// End-to-end against the real handlers: drives upload_start -> several
+// upload_chunk calls -> upload_finish exactly like the client does, and
+// checks the file on disk matches byte-for-byte. Exercises the real
+// state.File handle kept open across chunks (rather than the old
+// open/write/close-per-chunk code), so this would catch truncation,
+// overwriting, or interleaving bugs a build-only check can't.
+func TestUploadChunkRoundTrip(t *testing.T) {
+	setupTest(t)
+	state = &State{State: IDLE}
+
+	filename := "roundtrip-test.img.xz"
+	payload := bytes.Repeat([]byte("The quick brown fox jumps over the lazy dog. "), 5000) // ~225KB
+
+	startBody, _ := json.Marshal(map[string]any{
+		"filename":   filename,
+		"size":       len(payload),
+		"start_time": 0,
+	})
+	w := httptest.NewRecorder()
+	uploadStart(w, httptest.NewRequest("PUT", "/api/upload_start", bytes.NewReader(startBody)))
+	if state.File == nil {
+		t.Fatal("uploadStart did not open state.File")
+	}
+
+	const chunkSize = 10 * 1024 // small relative to payload so this genuinely exercises multiple sequential chunk writes, not just one
+	for i := 0; i < len(payload); i += chunkSize {
+		end := i + chunkSize
+		if end > len(payload) {
+			end = len(payload)
+		}
+		encoded := "data:application/octet-stream;base64," + base64.StdEncoding.EncodeToString(payload[i:end])
+		chunkBody, _ := json.Marshal(map[string]string{"chunk": encoded})
+		w := httptest.NewRecorder()
+		uploadChunk(w, httptest.NewRequest("POST", "/api/upload_chunk", bytes.NewReader(chunkBody)))
+
+		var resp map[string]bool
+		if err := json.Unmarshal(w.Body.Bytes(), &resp); err != nil {
+			t.Fatalf("chunk at offset %d: bad response body: %v", i, err)
+		}
+		if !resp["success"] {
+			t.Fatalf("chunk at offset %d: server reported failure", i)
+		}
+	}
+
+	uploadFinish(httptest.NewRecorder(), httptest.NewRequest("PUT", "/api/upload_finish", nil))
+
+	if state.File != nil {
+		t.Error("uploadFinish did not clear state.File")
+	}
+
+	got, err := os.ReadFile(filepath.Join(images_folder, filename))
+	if err != nil {
+		t.Fatalf("reading uploaded file: %v", err)
+	}
+	if !bytes.Equal(got, payload) {
+		t.Errorf("uploaded file mismatch: got %d bytes, want %d bytes", len(got), len(payload))
+	}
+	if state.BytesNow != len(payload) {
+		t.Errorf("state.BytesNow = %d, want %d", state.BytesNow, len(payload))
+	}
+}