start plop from grub

[Jim]

My system normally boots from /dev/sdd1.  I've installed Plop to 
/dev/sda1. If I switch on the machine and press F12 to select a boot 
device, then pick /dev/sda1, Plop loads.   When I installed Plop, I 
chose the MBR of /dev/sda1.


Is there a way to put a menu entry in Grub2 that would tell the system 
to boot from /dev/sda1?


I did manage to create a menu entry in /etc/grub.d/40_custom that lets 
me load plop, but when I do, it gives me a warning that I can't save 
changes.  Is there a way to fix it so I can save changes?  Here is my 
40_custom file.


#!/bin/sh
exec tail -n +3 $0
# This file provides an easy way to add custom menu entries. Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.


menuentry "Plop bootloader" {
insmod ext2
         set root='hd3,msdos1'
         if [ x$feature_platform_search_hint = xy ]; then
           search --no-floppy --fs-uuid --set=root 
--hint-bios=hd3,msdos1 --hint-efi=hd3,msdos1 
--hint-baremetal=ahci3,msdos1 15d4e39e-2e6c-4ee4-9a24-67fd9ccab8f9
         else
           search --no-floppy --fs-uuid --set=root 
15d4e39e-2e6c-4ee4-9a24-67fd9ccab8f9
         fi
         linux   /boot/vmlinuz-4.15.0-47-generic 
root=UUID=15d4e39e-2e6c-4ee4-9a24-67fd9ccab8f9
         linux16 /boot/plpbt.bin
}

thanks