[Techtalk] Device for boot loader installation

[David Sumbler]

I have changed the layout of the hard drive on my partner's computer,
using gparted as suggested by Akkana.  It now looks like this:

Device         Start        End    Sectors  Size Type
/dev/sda1       2048    1050623    1048576  512M EFI System
/dev/sda2    1050624   84936703   83886080   40G Linux filesystem
/dev/sda3  168822784  185599999   16777216    8G Linux swap
/dev/sda4   84936704  168822783   83886080   40G Linux filesystem
/dev/sda5  185600000 1953523711 1767923712  843G Linux filesystem

The partition numbers are not in disk order, but I gather that that
doesn't matter.

/dev/sda2 contains Ubuntu16.04
/dev/sda5 is mounted as /home

Everything is working fine.

I want to install Ubuntu20.04 on /dev/sda4

The Ubuntu installer (using a live USB stick) comes up with a screen
showing how the partitions should be used.  I tell it that the system
should be installed on /dev/sda4 and that /dev/sda5 should be mounted
as /home.

But if I tell it to use either /dev/sda or /dev/sda1 for the boot
loader, it comes up with a warning.

The fact is, I don't really understand where I should put it.  I
wouldn't have been quite sure with the older type of disk, and
certainly with the newer ones I really don't know what I am supposed to
do.  (It took me ages to work out why I couldn't create an extended
partition for partitions 4 and 5!)

The computer won't have any OS on it other than 2 Linux versions.  So
does it matter where I put the boot loader?  What are the pros and cons
of the 2 obvious choices, /dev/sda and /dev/sda1 ?

David