Trying again: I installed grub twice on two drives, how to fix it?

[Marco Fioretti]

On Sun, May 3, 2020 at 09:53, Ralf Mardorf via ubuntu-users  wrote:
I suspect you are misinterpreting something. Likely the GRUB packages
are installed on both installs and likely both installs contain GRUB
configuration files, but unlikely GRUB itself is installed on both
drives.

At first you could run

diff /boot/grub/grub.cfg /mount/point/install_2/boot/grub/grub.cfg
 Hi Ralf, and many thanks for your suggestion. Lockdown is stressing, I should have thought myself about changing the boot order...
Indeed, the old HD was higher in boot sequence than the new SSD one.
So I disabled the HD in the BIOS, rebooted, and this is the status now:
Both installs are still working, and still listed in grub menu, but inverted.
Now the SSD install is first, and chosen as default,  which is good, and the other is at the bottom.
The first lines of diffs between the two grub files are at the bottom. /boot/grub/grub.cfg is the one on
the SSD drive, which in fact also points at a newer kernel version (because after installing the SSD I
never ran apt-get update/upgrade on the HD install...)
Now I need to:
1)remove from grub the option to boot the HD install. What is the recommended way to do it, in a weird (for me now, at least) setup like this?
2) reconfigure/repartition the HD. Right now, what is still $HOME when booting the HD install is $HOME/home/marco in the SSD install, and this is how the HD is partitioned and mounted:
excerpt of fdisk -l
Disk /dev/sda: 298,9 GiB, 320072933376 bytes, 625142448 sectors
Disk model: HGST HTS725032A7
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0xceb4c5c7

Device     Boot    Start       End   Sectors   Size Id Type
/dev/sda1  *        2048    206847    204800   100M  7 HPFS/NTFS/exFAT
/dev/sda2         206848  82238097  82031250  39,1G  7 HPFS/NTFS/exFAT
/dev/sda3       82239488 625141759 542902272 258,9G 83 Linux
#> mount | grep home
/dev/sda3 on /home type ext4 (rw,relatime)
What I would need, instead, is: delete /dev/sda1 and /dev/sda2, expand (WITHOUT canceling anything it contains!!!) /dev/sda3 so it uses the whole HD disk, and mount it in the SSD install (which at that point would be the only one, of course) as /backup, or something similar. That way, I would do all my work on the faster SSD drive, and use the other as storage for backups, or stuff I seldom need, being able to replace it any time without touching the working SSD install.
How to proceed? Thanks (and expect unrelated questions to appear in the list, it is "Ubuntu overdue maintenance day" here, on several computers...)
marco
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