diff --git a/internal/backend/azure/azure.go b/internal/backend/azure/azure.go
index 53064702f..f0045c8aa 100644
--- a/internal/backend/azure/azure.go
+++ b/internal/backend/azure/azure.go
@@ -172,6 +172,7 @@ func (be *Backend) saveLarge(ctx context.Context, objName string, rd restic.Rewi
 	// read the data, in 100 MiB chunks
 	buf := make([]byte, 100*1024*1024)
 	var blocks []storage.Block
+	uploadedBytes := 0
 
 	for {
 		n, err := io.ReadFull(rd, buf)
@@ -188,6 +189,7 @@ func (be *Backend) saveLarge(ctx context.Context, objName string, rd restic.Rewi
 		}
 
 		buf = buf[:n]
+		uploadedBytes += n
 
 		// upload it as a new "block", use the base64 hash for the ID
 		h := restic.Hash(buf)
@@ -204,6 +206,11 @@ func (be *Backend) saveLarge(ctx context.Context, objName string, rd restic.Rewi
 		})
 	}
 
+	// sanity check
+	if uploadedBytes != int(rd.Length()) {
+		return errors.Errorf("wrote %d bytes instead of the expected %d bytes", uploadedBytes, rd.Length())
+	}
+
 	debug.Log("uploaded %d parts: %v", len(blocks), blocks)
 	err = file.PutBlockList(blocks, nil)
 	debug.Log("PutBlockList returned %v", err)
diff --git a/internal/backend/b2/b2.go b/internal/backend/b2/b2.go
index f6d4331c8..471ff1c62 100644
--- a/internal/backend/b2/b2.go
+++ b/internal/backend/b2/b2.go
@@ -209,6 +209,10 @@ func (be *b2Backend) Save(ctx context.Context, h restic.Handle, rd restic.Rewind
 		return errors.Wrap(err, "Copy")
 	}
 
+	// sanity check
+	if n != rd.Length() {
+		return errors.Errorf("wrote %d bytes instead of the expected %d bytes", n, rd.Length())
+	}
 	return errors.Wrap(w.Close(), "Close")
 }
 
diff --git a/internal/backend/gs/gs.go b/internal/backend/gs/gs.go
index 0b0cea8be..7e4ac0702 100644
--- a/internal/backend/gs/gs.go
+++ b/internal/backend/gs/gs.go
@@ -245,6 +245,10 @@ func (be *Backend) Save(ctx context.Context, h restic.Handle, rd restic.RewindRe
 	}
 
 	debug.Log("%v -> %v bytes", objName, wbytes)
+	// sanity check
+	if wbytes != rd.Length() {
+		return errors.Errorf("wrote %d bytes instead of the expected %d bytes", wbytes, rd.Length())
+	}
 	return nil
 }
 
diff --git a/internal/backend/local/local.go b/internal/backend/local/local.go
index c195bdc07..f1b658864 100644
--- a/internal/backend/local/local.go
+++ b/internal/backend/local/local.go
@@ -118,11 +118,16 @@ func (b *Local) Save(ctx context.Context, h restic.Handle, rd restic.RewindReade
 	}
 
 	// save data, then sync
-	_, err = io.Copy(f, rd)
+	wbytes, err := io.Copy(f, rd)
 	if err != nil {
 		_ = f.Close()
 		return errors.Wrap(err, "Write")
 	}
+	// sanity check
+	if wbytes != rd.Length() {
+		_ = f.Close()
+		return errors.Errorf("wrote %d bytes instead of the expected %d bytes", wbytes, rd.Length())
+	}
 
 	if err = f.Sync(); err != nil {
 		pathErr, ok := err.(*os.PathError)
diff --git a/internal/backend/mem/mem_backend.go b/internal/backend/mem/mem_backend.go
index 8378630fa..950ae122d 100644
--- a/internal/backend/mem/mem_backend.go
+++ b/internal/backend/mem/mem_backend.go
@@ -84,6 +84,11 @@ func (be *MemoryBackend) Save(ctx context.Context, h restic.Handle, rd restic.Re
 	be.data[h] = buf
 	debug.Log("saved %v bytes at %v", len(buf), h)
 
+	// sanity check
+	if int64(len(buf)) != rd.Length() {
+		return errors.Errorf("wrote %d bytes instead of the expected %d bytes", len(buf), rd.Length())
+	}
+
 	return ctx.Err()
 }
 
diff --git a/internal/backend/s3/s3.go b/internal/backend/s3/s3.go
index 83d784813..8a8494623 100644
--- a/internal/backend/s3/s3.go
+++ b/internal/backend/s3/s3.go
@@ -272,9 +272,14 @@ func (be *Backend) Save(ctx context.Context, h restic.Handle, rd restic.RewindRe
 	opts.ContentType = "application/octet-stream"
 
 	debug.Log("PutObject(%v, %v, %v)", be.cfg.Bucket, objName, rd.Length())
-	n, err := be.client.PutObject(ctx, be.cfg.Bucket, objName, ioutil.NopCloser(rd), int64(rd.Length()), opts)
+	info, err := be.client.PutObject(ctx, be.cfg.Bucket, objName, ioutil.NopCloser(rd), int64(rd.Length()), opts)
 
-	debug.Log("%v -> %v bytes, err %#v: %v", objName, n, err, err)
+	debug.Log("%v -> %v bytes, err %#v: %v", objName, info.Size, err, err)
+
+	// sanity check
+	if err != nil && info.Size != rd.Length() {
+		return errors.Errorf("wrote %d bytes instead of the expected %d bytes", info.Size, rd.Length())
+	}
 
 	return errors.Wrap(err, "client.PutObject")
 }
diff --git a/internal/backend/sftp/sftp.go b/internal/backend/sftp/sftp.go
index 7fa4b7319..3d5ee3ba4 100644
--- a/internal/backend/sftp/sftp.go
+++ b/internal/backend/sftp/sftp.go
@@ -288,12 +288,18 @@ func (r *SFTP) Save(ctx context.Context, h restic.Handle, rd restic.RewindReader
 	}
 
 	// save data, make sure to use the optimized sftp upload method
-	_, err = f.ReadFrom(rd)
+	wbytes, err := f.ReadFrom(rd)
 	if err != nil {
 		_ = f.Close()
 		return errors.Wrap(err, "Write")
 	}
 
+	// sanity check
+	if wbytes != rd.Length() {
+		_ = f.Close()
+		return errors.Errorf("wrote %d bytes instead of the expected %d bytes", wbytes, rd.Length())
+	}
+
 	err = f.Close()
 	if err != nil {
 		return errors.Wrap(err, "Close")