diff --git a/src/add-ons/kernel/partitioning_systems/intel/PartitionMap.h b/src/add-ons/kernel/partitioning_systems/intel/PartitionMap.h
index 647e10c6be..56fdc2b913 100644
--- a/src/add-ons/kernel/partitioning_systems/intel/PartitionMap.h
+++ b/src/add-ons/kernel/partitioning_systems/intel/PartitionMap.h
@@ -75,7 +75,9 @@ struct partition_descriptor {
 
 // partition_table
 struct partition_table {
-	char					code_area[446];
+	char					code_area[440];
+	uint32					disk_id;
+	uint16					reserved;
 	partition_descriptor	table[4];
 	uint16					signature;
 
diff --git a/src/add-ons/kernel/partitioning_systems/intel/PartitionMapWriter.cpp b/src/add-ons/kernel/partitioning_systems/intel/PartitionMapWriter.cpp
index 3b966f8018..3f9be03b3c 100644
--- a/src/add-ons/kernel/partitioning_systems/intel/PartitionMapWriter.cpp
+++ b/src/add-ons/kernel/partitioning_systems/intel/PartitionMapWriter.cpp
@@ -10,11 +10,14 @@
 
 #include <errno.h>
 #include <stdio.h>
-#include <unistd.h>
 #include <string.h>
+#include <unistd.h>
 
 #include <new>
 
+#include <debug.h>
+
+
 #ifndef _USER_MODE
 #	include <KernelExport.h>
 #endif
@@ -134,13 +137,14 @@ PartitionMapWriter::WriteMBR(const PartitionMap* map, bool writeBootCode)
 		return B_BAD_VALUE;
 
 	partition_table partitionTable;
+	status_t error = _ReadBlock(0, partitionTable);
+	if (error != B_OK)
+		return error;
 	if (writeBootCode) {
+		// the boot code must be small enough to fit in the code area
+		STATIC_ASSERT(sizeof(kBootCode) <= sizeof(partitionTable.code_area));
 		partitionTable.clear_code_area();
 		partitionTable.fill_code_area(kBootCode, sizeof(kBootCode));
-	} else {
-		status_t error = _ReadBlock(0, partitionTable);
-		if (error != B_OK)
-			return error;
 	}
 
 	partitionTable.signature = kPartitionTableSectorSignature;
@@ -152,7 +156,7 @@ PartitionMapWriter::WriteMBR(const PartitionMap* map, bool writeBootCode)
 		partition->GetPartitionDescriptor(descriptor);
 	}
 
-	status_t error = _WriteBlock(0, partitionTable);
+	error = _WriteBlock(0, partitionTable);
 	return error;
 }