diff --git a/bootloader_message/bootloader_message.cpp b/bootloader_message/bootloader_message.cpp
index 4d1ce5b..449f40e 100644
--- a/bootloader_message/bootloader_message.cpp
+++ b/bootloader_message/bootloader_message.cpp
@@ -159,7 +159,10 @@
     *err = "no misc device set";
     return false;
   }
-  int fd = (open(misc_blk_device.c_str(), O_WRONLY | O_SYNC));
+  int open_flags = O_WRONLY | O_SYNC;
+  if (offset > 0)
+    open_flags = O_RDWR | O_APPEND | O_SYNC;
+  int fd = (open(misc_blk_device.c_str(), open_flags));
   if (fd == -1) {
     *err = "failed to open " + misc_blk_device + ": ";
     *err += strerror(errno);