diff --git a/osbase/fat32/utils.go b/osbase/fat32/utils.go
index 8ddb162..309ecf6 100644
--- a/osbase/fat32/utils.go
+++ b/osbase/fat32/utils.go
@@ -11,8 +11,6 @@
 
 // Wraps a writer and provides support for writing padding up to a specified
 // alignment.
-// TODO(lorenz): Implement WriterTo when w implements it to allow for copy
-// offload
 type blockWriter struct {
 	w io.Writer
 	n int64
@@ -28,6 +26,13 @@
 	return
 }
 
+func (b *blockWriter) ReadFrom(r io.Reader) (n int64, err error) {
+	// io.Copy forwards to b.w.ReadFrom if available.
+	n, err = io.Copy(b.w, r)
+	b.n += n
+	return
+}
+
 func (b *blockWriter) FinishBlock(alignment int64, mustZero bool) (err error) {
 	requiredBytes := (alignment - (b.n % alignment)) % alignment
 	if requiredBytes == 0 {