the problem is that gdubenchmarkdialog.c uses signed 32bit lseek(3)
prototype to seek the disk position and fails on offsets further than
2^31.
the fix is to switch to 64bit prototype lseek64(3) or just ensure that
off_t is of 64bit type.
here is a (trivial) patch:
diff --git a/src/disks/gdubenchmarkdialog.c
b/src/disks/gdubenchmarkdialog.c
index 6f751da..e47d7b6 100644
--- a/src/disks/gdubenchmarkdialog.c
+++ b/src/disks/gdubenchmarkdialog.c
@@ -9,6 +9,7 @@
#include "config.h"
+#define _FILE_OFFSET_BITS 64
#include <glib/gi18n.h>
#include <gio/gunixfdlist.h>
#include <gio/gunixinputstream.h>