Hi all

I created a dual boot, first had Kubuntu (SDA5), then installed Manjaro KDE in the 2nd partitions (SDA6,7,8), of quick the Boot Grub menu is now from Manjaro.

But now I wish to remove Manjaro and return to just a single boot setup. If possible, how can I safely reclaim booting from the original Kubuntu Grub boot menu?

I'm worried that if removing Manjaro (probably by formatting the partition's while in Kubuntu), that this would destroy any boot capability and I'll be unable to get back into Kubuntu.
My boot system is MBR, old style bios on this machine, if that makes any difference.

I'm kinda new to linux ... so heavens I'm praying there's an easy quick solution.

Thanks in advance
Rob

sda 8:0 0 223.6G 0 disk
├─sda1 8:1 0 512M 0 part /boot/efi
├─sda2 8:2 0 1K 0 part
├─sda5 8:5 0 125.4G 0 part /
├─sda6 8:6 0 29.3G 0 part
├─sda7 8:7 0 56.7G 0 part
└─sda8 8:8 0 11.7G 0 part

Operating System: Kubuntu 20.04
KDE Plasma Version: 5.18.5
KDE Frameworks Version: 5.68.0
Qt Version: 5.12.8
Kernel Version: 5.11.0-41-generic
OS Type: 64-bit
Processors: 8 × Intel® Core™ i7-2630QM CPU @ 2.00GHz
Memory: 7,7 GiB of RAM

Is certainly makes a difference, and with an EFI partition seems doubtful. Let's have some more data before surgery. Run these commands from a terminal and post results inside [code] tags to keep the alignment so it is readable.Changing the boot back is straightforward, but different for each.
Forgot to mention - sudo will require your password; it won't show anything as you type it.

Boot into Kubuntu. Then install Grub from within Kubuntu. It will probably dual boot Kubuntu and Manjaro with Kubunto as the default. After you format the Manjaro
partition you can install Grub again from Kubuntu and the second time Grub will only find the Kubuntu partition.

If /etc/default/grub contains GRUB_DISABLE_OS_PROBER="true", then reinstalling Grub from Kubuntu will not include any other OS in Grub's boot menu. Just make that edit if necessary, then run sudo update-grub from Kubuntu boot, reboot to verify you're in Kubuntu's Grub menu.

Before doing the above sequence, follow syg00's instructions, in order to understand whether you're dealing with UEFI vs. legacy booting issues. If Kubuntu is installed in UEFI mode, then it should be enough to use efibootmgr to ensure Grub is first in boot priority. If Manjaro was installed in legacy mode, it may be necessary or desirable to go into the BIOS and disable CSM booting.

For the future, if you ever wish to add another Linux distro, install the additional distro without any bootloader. This will maintain the existing boot configuration, which can be post-installation updated to add the new distro to its menu via GRUB_DISABLE_OS_PROBER="false", or commenting out or removing GRUB_DISABLE_OS_PROBER="true", followed by sudo update-grub in Kubuntu.

Sorry for late reply on this ... only found the thread now again, new here.
On this day I decided to just reinstall. Saved a lot of hassle. Kind of!

Thanks for assistance