It's not a big deal since there is no error reported so just warning.

But, if you wish to get rid of this, you need to wipe sector 32. To do this you have many choices:

a. Wiping the whole hard disk;

b. writing zeros to Whole sectors in your MBR and reinstall your grub;

c. Writing zeros to sector 32 in your MBR (that's what we'll do here).

To do that follow the following steps:

1. Backup your MBR:

   
   
   

   sudo dd if=/dev/sda of=~/first_63_sectors bs=512 count=63
   
   




2. zero your sector number 32:

   
   
   

   sudo dd if=/dev/zero of=/dev/sda bs=512 count=1 seek=32
   
   




3. chroot and reinstall grub:
   Here "/media" should be whatever your chosen mount point is. Some step-by-step instructions use "/mnt" rather than "/media".

   
   
   

   sudo mount /dev/sda* /media/sda*
   
   
   
   sudo mount --bind /dev /media/sda*/dev
   
   
   
   sudo mount --bind /proc /media/sda*/proc
   
   
   
   sudo mount --bind /sys /media/sda*/sys
   
   
   
   sudo chroot /media/sda*
   
   

   
   
   

   PS: replace * by the appropriate number of your hdd

   
   
   

   Now update your grub:

   
   
   

   sudo update-grub
   
   

Note: You are not obliged to these steps, since its not an error that may affect your system.