diff --git a/backend/pikpak/helper.go b/backend/pikpak/helper.go
index 9bc8ea041..ee0327553 100644
--- a/backend/pikpak/helper.go
+++ b/backend/pikpak/helper.go
@@ -1,7 +1,6 @@
 package pikpak
 
 import (
-	"bytes"
 	"context"
 	"crypto/md5"
 	"crypto/sha1"
@@ -23,6 +22,7 @@ import (
 	"github.com/rclone/rclone/fs"
 	"github.com/rclone/rclone/fs/config/configmap"
 	"github.com/rclone/rclone/fs/fserrors"
+	"github.com/rclone/rclone/lib/multipart"
 	"github.com/rclone/rclone/lib/rest"
 )
 
@@ -307,7 +307,9 @@ func readGcid(in io.Reader, size, threshold int64) (gcid string, out io.Reader,
 	cleanup = func() {}
 
 	// don't cache small files on disk to reduce wear of the disk
-	if size > threshold {
+	//
+	// files of unknown size go to disk as they may not fit in memory
+	if size < 0 || size > threshold {
 		var tempFile *os.File
 
 		// create the cache file
@@ -340,13 +342,16 @@ func readGcid(in io.Reader, size, threshold int64) (gcid string, out io.Reader,
 		// replace the already read source with a reader of our cached file
 		out = tempFile
 	} else {
-		buf := &bytes.Buffer{}
-		teeReader := io.TeeReader(in, buf)
+		rw := multipart.NewRW()
+		cleanup = func() {
+			_ = rw.Close()
+		}
+		teeReader := io.TeeReader(in, rw)
 
 		if gcid, err = calcGcid(teeReader, size); err != nil {
 			return
 		}
-		out = buf
+		out = rw
 	}
 	return
 }
diff --git a/backend/pikpak/helper_internal_test.go b/backend/pikpak/helper_internal_test.go
new file mode 100644
index 000000000..1384a756d
--- /dev/null
+++ b/backend/pikpak/helper_internal_test.go
@@ -0,0 +1,46 @@
+package pikpak
+
+import (
+	"bytes"
+	"io"
+	"os"
+	"testing"
+
+	"github.com/rclone/rclone/lib/pool"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestReadGcid checks readGcid hashes and replays its input from a pooled
+// buffer for small known sizes, and from a temp file for large or unknown
+// sizes, releasing the buffer on cleanup.
+func TestReadGcid(t *testing.T) {
+	content := bytes.Repeat([]byte("pikpak"), 1024)
+	want, err := calcGcid(bytes.NewReader(content), int64(len(content)))
+	require.NoError(t, err)
+
+	for _, test := range []struct {
+		name      string
+		size      int64
+		threshold int64
+		wantFile  bool
+	}{
+		{"memory", int64(len(content)), int64(len(content)), false},
+		{"file", int64(len(content)), int64(len(content)) - 1, true},
+		{"unknown size", -1, int64(len(content)), true},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			inUse := pool.Global().InUse()
+			gcid, out, cleanup, err := readGcid(bytes.NewReader(content), test.size, test.threshold)
+			require.NoError(t, err)
+			assert.Equal(t, want, gcid)
+			_, isFile := out.(*os.File)
+			assert.Equal(t, test.wantFile, isFile, "expected temp file %v", test.wantFile)
+			got, err := io.ReadAll(out)
+			require.NoError(t, err)
+			assert.Equal(t, content, got)
+			cleanup()
+			assert.Equal(t, inUse, pool.Global().InUse(), "buffer should be returned to the pool")
+		})
+	}
+}
diff --git a/backend/pikpak/pikpak.go b/backend/pikpak/pikpak.go
index df5c3f499..a765efa06 100644
--- a/backend/pikpak/pikpak.go
+++ b/backend/pikpak/pikpak.go
@@ -224,7 +224,7 @@ Fill in for rclone to use a non root folder as its starting point.
 			Advanced: true,
 		}, {
 			Name:     "hash_memory_limit",
-			Help:     "Files bigger than this will be cached on disk to calculate hash if required.",
+			Help:     "Files bigger than this will be cached on disk to calculate hash if required.\n\nFiles of unknown size are always cached on disk.",
 			Default:  fs.SizeSuffix(10 * 1024 * 1024),
 			Advanced: true,
 		}, {