diff --git a/test/sftp/basic_test.go b/test/sftp/basic_test.go
index bad875208..17f3b191f 100644
--- a/test/sftp/basic_test.go
+++ b/test/sftp/basic_test.go
@@ -3,6 +3,7 @@ package sftp
 import (
 	"bytes"
 	"io"
+	"os"
 	"path"
 	"testing"
 
@@ -602,6 +603,32 @@ func TestFileContent(t *testing.T) {
 		sftpClient.Remove("/binary.bin")
 	})
 
+	t.Run("WriteAtOffsetKeepsExistingBytes", func(t *testing.T) {
+		file, err := sftpClient.Create("/resume.txt")
+		require.NoError(t, err)
+		_, err = file.Write([]byte("hello world"))
+		require.NoError(t, err)
+		require.NoError(t, file.Close())
+
+		// Reopen without O_TRUNC and write past the start, as a resumed upload does
+		file, err = sftpClient.OpenFile("/resume.txt", os.O_WRONLY)
+		require.NoError(t, err)
+		_, err = file.WriteAt([]byte("WORLD!"), 6)
+		require.NoError(t, err)
+		require.NoError(t, file.Close())
+
+		readFile, err := sftpClient.Open("/resume.txt")
+		require.NoError(t, err)
+		content, err := io.ReadAll(readFile)
+		require.NoError(t, err)
+		readFile.Close()
+
+		require.Equal(t, "hello WORLD!", string(content))
+
+		// Clean up
+		sftpClient.Remove("/resume.txt")
+	})
+
 	t.Run("EmptyFile", func(t *testing.T) {
 		file, err := sftpClient.Create("/empty.txt")
 		require.NoError(t, err)
diff --git a/weed/filer/filechunk_manifest.go b/weed/filer/filechunk_manifest.go
index a8661549c..3eacae5cd 100644
--- a/weed/filer/filechunk_manifest.go
+++ b/weed/filer/filechunk_manifest.go
@@ -458,13 +458,13 @@ func retriedStreamFetchChunkData(ctx context.Context, writer io.Writer, urlStrin
 				localProcessed += writtenCount
 				totalWritten += writtenCount
 			})
-			if !shouldRetry {
-				break
-			}
 			if writeErr != nil {
 				err = writeErr
 				break
 			}
+			if !shouldRetry {
+				break
+			}
 			if err != nil {
 				failed = true
 				glog.V(0).InfofCtx(ctx, "read %s failed, err: %v", urlString, err)
diff --git a/weed/filer/filechunk_manifest_test.go b/weed/filer/filechunk_manifest_test.go
index e23bf5366..5e82c7045 100644
--- a/weed/filer/filechunk_manifest_test.go
+++ b/weed/filer/filechunk_manifest_test.go
@@ -787,3 +787,23 @@ func TestFetchWholeChunkUsesProvidedFilerJwt(t *testing.T) {
 		return ""
 	}))
 }
+
+// A completed fetch whose destination write failed is a failed copy, not a
+// short success: the caller must see the write error, or a temp disk filling
+// mid-copy would still upload the truncated file.
+func TestRetriedStreamFetchChunkDataReportsWriteError(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Write([]byte("0123456789"))
+	}))
+	t.Cleanup(srv.Close)
+
+	written, err := retriedStreamFetchChunkData(context.Background(), &failOnWrite{}, []string{srv.URL + "/5,abc"}, "", nil, false, true, 0, 0, nil)
+	assert.Error(t, err)
+	assert.Zero(t, written)
+}
+
+type failOnWrite struct{}
+
+func (w *failOnWrite) Write(p []byte) (int, error) {
+	return 0, fmt.Errorf("no space left")
+}
diff --git a/weed/sftpd/sftp_copy_existing_test.go b/weed/sftpd/sftp_copy_existing_test.go
new file mode 100644
index 000000000..5f603107a
--- /dev/null
+++ b/weed/sftpd/sftp_copy_existing_test.go
@@ -0,0 +1,210 @@
+package sftpd
+
+import (
+	"bytes"
+	"context"
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/seaweedfs/seaweedfs/weed/pb"
+	"github.com/seaweedfs/seaweedfs/weed/pb/filer_pb"
+	"github.com/seaweedfs/seaweedfs/weed/sftpd/user"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/credentials/insecure"
+	"google.golang.org/grpc/status"
+)
+
+type copyTestFiler struct {
+	filer_pb.UnimplementedSeaweedFilerServer
+	entries   map[string]*filer_pb.Entry
+	volumeUrl string
+}
+
+func (f *copyTestFiler) LookupDirectoryEntry(_ context.Context, req *filer_pb.LookupDirectoryEntryRequest) (*filer_pb.LookupDirectoryEntryResponse, error) {
+	if entry, ok := f.entries[req.Directory+"/"+req.Name]; ok {
+		return &filer_pb.LookupDirectoryEntryResponse{Entry: entry}, nil
+	}
+	return nil, status.Error(codes.NotFound, "no entry is found")
+}
+
+func (f *copyTestFiler) LookupVolume(_ context.Context, req *filer_pb.LookupVolumeRequest) (*filer_pb.LookupVolumeResponse, error) {
+	resp := &filer_pb.LookupVolumeResponse{LocationsMap: make(map[string]*filer_pb.Locations)}
+	for _, vid := range req.VolumeIds {
+		resp.LocationsMap[vid] = &filer_pb.Locations{Locations: []*filer_pb.Location{{Url: f.volumeUrl}}}
+	}
+	return resp, nil
+}
+
+func newCopyTestServer(t *testing.T, volumeUrl string, entries map[string]*filer_pb.Entry) *SftpServer {
+	lis, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("listen: %v", err)
+	}
+	grpcServer := grpc.NewServer()
+	filer_pb.RegisterSeaweedFilerServer(grpcServer, &copyTestFiler{entries: entries, volumeUrl: volumeUrl})
+	go func() { _ = grpcServer.Serve(lis) }()
+	t.Cleanup(grpcServer.Stop)
+
+	_, port, _ := net.SplitHostPort(lis.Addr().String())
+	return &SftpServer{
+		filerAddr:      pb.ServerAddress("127.0.0.1:0." + port),
+		grpcDialOption: grpc.WithTransportCredentials(insecure.NewCredentials()),
+		user:           &user.User{Username: "alice", Uid: 1000, HomeDir: "/"},
+	}
+}
+
+func chunkedEntry(size int64, chunks ...*filer_pb.FileChunk) *filer_pb.Entry {
+	return &filer_pb.Entry{
+		Attributes:  &filer_pb.FuseAttributes{FileSize: uint64(size)},
+		Chunks:      chunks,
+		IsDirectory: false,
+	}
+}
+
+func fileChunk(fileId string, offset, size int64) *filer_pb.FileChunk {
+	return &filer_pb.FileChunk{FileId: fileId, Offset: offset, Size: uint64(size), ModifiedTsNs: 1}
+}
+
+// A non-truncating write must keep untouched bytes, including a sparse file's
+// zeros between chunks. Stopping at the first gap would drop the tail.
+func TestCopyExistingContentFillsSparseGaps(t *testing.T) {
+	chunkA := []byte("AAAAAAAAAA")
+	chunkB := []byte("BBBBBBBBBB")
+	objects := map[string][]byte{"3,01": chunkA, "3,02": chunkB}
+	volume := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		data, ok := objects[r.URL.Path[1:]]
+		if !ok {
+			http.Error(w, "not found", http.StatusNotFound)
+			return
+		}
+		_, _ = w.Write(data)
+	}))
+	t.Cleanup(volume.Close)
+
+	entries := map[string]*filer_pb.Entry{
+		"/home/sparse.bin": chunkedEntry(100, fileChunk("3,01", 0, 10), fileChunk("3,02", 90, 10)),
+	}
+	fs := newCopyTestServer(t, volume.Listener.Addr().String(), entries)
+
+	var dst bytes.Buffer
+	if err := fs.copyExistingContent("/home/sparse.bin", &dst); err != nil {
+		t.Fatalf("copyExistingContent: %v", err)
+	}
+	want := append(append(append([]byte{}, chunkA...), make([]byte, 80)...), chunkB...)
+	if !bytes.Equal(dst.Bytes(), want) {
+		t.Fatalf("copied %d bytes, want %d with a zero-filled gap; tail present: %v", dst.Len(), len(want), bytes.Contains(dst.Bytes(), chunkB))
+	}
+}
+
+// A remote-only file has no local chunks: copying nothing would upload an
+// empty replacement on close. Refusing the read keeps the remote object.
+func TestCopyExistingContentRejectsUnreadableEntry(t *testing.T) {
+	remoteOnly := chunkedEntry(0)
+	remoteOnly.RemoteEntry = &filer_pb.RemoteEntry{RemoteSize: 100}
+	entries := map[string]*filer_pb.Entry{
+		"/home/remote.bin": remoteOnly,
+	}
+	fs := newCopyTestServer(t, "", entries)
+
+	var dst bytes.Buffer
+	if err := fs.copyExistingContent("/home/remote.bin", &dst); err == nil {
+		t.Fatal("expected refusal when no local bytes exist")
+	}
+}
+
+// A locally shortened file still names its older, larger remote object. Once
+// the local entry is newer the remote size is stale: copying must not pad the
+// file back out to it.
+func TestCopyExistingContentIgnoresStaleRemoteSize(t *testing.T) {
+	chunkA := []byte("AAAAAAAAAA")
+	volume := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/3,01" {
+			http.Error(w, "not found", http.StatusNotFound)
+			return
+		}
+		_, _ = w.Write(chunkA)
+	}))
+	t.Cleanup(volume.Close)
+
+	entries := map[string]*filer_pb.Entry{
+		"/home/short.bin": {
+			Attributes:  &filer_pb.FuseAttributes{FileSize: 10, Mtime: 200},
+			Chunks:      []*filer_pb.FileChunk{fileChunk("3,01", 0, 10)},
+			RemoteEntry: &filer_pb.RemoteEntry{RemoteSize: 100, RemoteMtime: 100},
+		},
+	}
+	fs := newCopyTestServer(t, volume.Listener.Addr().String(), entries)
+
+	var dst bytes.Buffer
+	if err := fs.copyExistingContent("/home/short.bin", &dst); err != nil {
+		t.Fatalf("copyExistingContent: %v", err)
+	}
+	if !bytes.Equal(dst.Bytes(), chunkA) {
+		t.Fatalf("copied %d bytes, want the %d local bytes only", dst.Len(), len(chunkA))
+	}
+}
+
+// A file whose size is all holes still keeps its zeros.
+func TestCopyExistingContentAllHoles(t *testing.T) {
+	entries := map[string]*filer_pb.Entry{
+		"/home/holes.bin": chunkedEntry(10),
+	}
+	fs := newCopyTestServer(t, "", entries)
+
+	var dst bytes.Buffer
+	if err := fs.copyExistingContent("/home/holes.bin", &dst); err != nil {
+		t.Fatalf("all-holes file: %v", err)
+	}
+	if dst.Len() != 10 || !bytes.Equal(dst.Bytes(), make([]byte, 10)) {
+		t.Fatalf("wrote %d bytes, want 10 zeros", dst.Len())
+	}
+}
+
+func TestCopyExistingContentInlineAndMissing(t *testing.T) {
+	entries := map[string]*filer_pb.Entry{
+		"/home/inline.txt": {Attributes: &filer_pb.FuseAttributes{FileSize: 5}, Content: []byte("hello")},
+	}
+	fs := newCopyTestServer(t, "", entries)
+
+	var dst bytes.Buffer
+	if err := fs.copyExistingContent("/home/inline.txt", &dst); err != nil {
+		t.Fatalf("inline content: %v", err)
+	}
+	if got := dst.String(); got != "hello" {
+		t.Fatalf("inline content = %q, want %q", got, "hello")
+	}
+
+	dst.Reset()
+	if err := fs.copyExistingContent("/home/missing.txt", &dst); err != nil || dst.Len() != 0 {
+		t.Fatalf("missing file: err=%v written=%d", err, dst.Len())
+	}
+}
+
+func TestCopyExistingContentTrailingGap(t *testing.T) {
+	chunkA := []byte("AAAAAAAAAA")
+	volume := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/3,01" {
+			http.Error(w, "not found", http.StatusNotFound)
+			return
+		}
+		_, _ = w.Write(chunkA)
+	}))
+	t.Cleanup(volume.Close)
+
+	entries := map[string]*filer_pb.Entry{
+		"/home/trailing.bin": chunkedEntry(50, fileChunk("3,01", 0, 10)),
+	}
+	fs := newCopyTestServer(t, volume.Listener.Addr().String(), entries)
+
+	var dst bytes.Buffer
+	if err := fs.copyExistingContent("/home/trailing.bin", &dst); err != nil {
+		t.Fatalf("copyExistingContent: %v", err)
+	}
+	want := append(append([]byte{}, chunkA...), make([]byte, 40)...)
+	if !bytes.Equal(dst.Bytes(), want) {
+		t.Fatalf("copied %d bytes, want %d with a zero-filled tail", dst.Len(), len(want))
+	}
+}
diff --git a/weed/sftpd/sftp_filer.go b/weed/sftpd/sftp_filer.go
index eb4a989a6..1262b75d8 100644
--- a/weed/sftpd/sftp_filer.go
+++ b/weed/sftpd/sftp_filer.go
@@ -15,6 +15,7 @@ import (
 	"time"
 
 	"github.com/pkg/sftp"
+	"github.com/seaweedfs/seaweedfs/weed/filer"
 	"github.com/seaweedfs/seaweedfs/weed/glog"
 	"github.com/seaweedfs/seaweedfs/weed/pb"
 	filer_pb "github.com/seaweedfs/seaweedfs/weed/pb/filer_pb"
@@ -23,6 +24,7 @@ import (
 	"github.com/seaweedfs/seaweedfs/weed/sftpd/user"
 	"github.com/seaweedfs/seaweedfs/weed/util"
 	util_http "github.com/seaweedfs/seaweedfs/weed/util/http"
+	"github.com/seaweedfs/seaweedfs/weed/wdclient"
 	"google.golang.org/grpc"
 )
 
@@ -159,6 +161,14 @@ func (fs *SftpServer) newFileWriter(r *sftp.Request) (io.WriterAt, error) {
 	if err != nil {
 		return nil, fmt.Errorf("failed to create temp file: %w", err)
 	}
+	// Without O_TRUNC the client writes into the existing file, so start from its current bytes.
+	if !r.Pflags().Trunc {
+		if err := fs.copyExistingContent(absPath, tmpFile); err != nil {
+			tmpFile.Close()
+			os.Remove(tmpFile.Name())
+			return nil, err
+		}
+	}
 
 	return &SeaweedSftpFileWriter{
 		fs:          *fs,
@@ -172,6 +182,75 @@ func (fs *SftpServer) newFileWriter(r *sftp.Request) (io.WriterAt, error) {
 	}, nil
 }
 
+func (fs *SftpServer) copyExistingContent(absPath string, dst io.Writer) error {
+	entry, err := fs.getEntry(absPath)
+	if err == os.ErrNotExist {
+		return nil
+	}
+	if err != nil {
+		return err
+	}
+	if entry.IsDirectory {
+		return nil
+	}
+	size := int64(filer.FileSize(entry))
+	remote := entry.GetRemoteEntry()
+	if len(entry.Content) == 0 && len(entry.GetChunks()) == 0 {
+		if remote != nil && remote.RemoteSize > 0 {
+			return fmt.Errorf("existing content of %s is not readable", absPath)
+		}
+		if size == 0 {
+			return nil
+		}
+		return writeZeros(dst, size)
+	}
+	if size == 0 {
+		return nil
+	}
+	if len(entry.Content) > 0 {
+		if _, err := dst.Write(entry.Content); err != nil {
+			return err
+		}
+		if remaining := size - int64(len(entry.Content)); remaining > 0 {
+			return writeZeros(dst, remaining)
+		}
+		return nil
+	}
+	streamFn, err := filer.PrepareStreamContent(lookupFileIdFn(filer.LookupFn(fs)), filer.JwtForVolumeServer, entry.GetChunks(), 0, size)
+	if err != nil {
+		return fmt.Errorf("read existing content of %s: %w", absPath, err)
+	}
+	if err := streamFn(dst); err != nil {
+		return fmt.Errorf("read existing content of %s: %w", absPath, err)
+	}
+	return nil
+}
+
+// lookupFileIdFn adapts a filer LookupFn to the chunk streamer's provider
+// interface. PrepareStreamContent fills unstored gaps with zeros, so a sparse
+// file's tail survives the copy.
+type lookupFileIdFn wdclient.LookupFileIdFunctionType
+
+func (fn lookupFileIdFn) GetLookupFileIdFunction() wdclient.LookupFileIdFunctionType {
+	return wdclient.LookupFileIdFunctionType(fn)
+}
+
+func writeZeros(w io.Writer, size int64) error {
+	zeros := make([]byte, 8192)
+	for size > 0 {
+		n := int64(len(zeros))
+		if size < n {
+			n = size
+		}
+		written, err := w.Write(zeros[:n])
+		size -= int64(written)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
 func (fs *SftpServer) removeEntry(absPath string) error {
 	return fs.deleteEntry(absPath, false)
 }