Index: base/setup/usetup/interface/usetup.c
===================================================================
--- base/setup/usetup/interface/usetup.c	(revision 74470)
+++ base/setup/usetup/interface/usetup.c	(working copy)
@@ -4417,6 +4417,7 @@
 {
     UCHAR PartitionType;
     NTSTATUS Status;
+    WCHAR OriginalDevicePathBuffer[MAX_PATH];
     WCHAR DestinationDevicePathBuffer[MAX_PATH];
     WCHAR SourceMbrPathBuffer[MAX_PATH];
     WCHAR DstPath[MAX_PATH];
@@ -4435,6 +4436,9 @@
     }
 
     /* Step 2: Write the MBR */
+    swprintf(OriginalDevicePathBuffer,
+             L"\\Device\\Harddisk%d\\Partition0",
+             PartitionList->OriginalSystemDisk->DiskNumber);
     swprintf(DestinationDevicePathBuffer,
              L"\\Device\\Harddisk%d\\Partition0",
              PartitionList->SystemDisk->DiskNumber);
@@ -4445,14 +4449,20 @@
     DPRINT1("Install MBR bootcode: %S ==> %S\n",
             SourceMbrPathBuffer, DestinationDevicePathBuffer);
 
-    if (IsThereAValidBootSector(DestinationDevicePathBuffer))
+    /*
+     * We need to save original MBR only if system disk is the same and already contains valid MBR
+     * This is just for reference to user if he wants to write back his old MBR back, because we
+     * are overwriting it, otherwise it is not used.
+     */
+    if (IsThereAValidBootSector(OriginalDevicePathBuffer) && 
+        PartitionList->OriginalSystemDisk->DiskNumber == PartitionList->SystemDisk->DiskNumber)
     {
         /* Save current MBR */
         wcscpy(DstPath, SystemRootPath.Buffer);
         wcscat(DstPath, L"\\mbr.old");
 
-        DPRINT1("Save MBR: %S ==> %S\n", DestinationDevicePathBuffer, DstPath);
-        Status = SaveBootSector(DestinationDevicePathBuffer, DstPath, sizeof(PARTITION_SECTOR));
+        DPRINT1("Save MBR: %S ==> %S\n", OriginalDevicePathBuffer, DstPath);
+        Status = SaveBootSector(OriginalDevicePathBuffer, DstPath, sizeof(PARTITION_SECTOR));
         if (!NT_SUCCESS(Status))
         {
             DPRINT1("SaveBootSector() failed (Status %lx)\n", Status);