I will try to explain you a few more things and maybe in more details.
Example ( i had to edit this part)
You have two drive and make partitions on it.
One disk is structured to MBR (msdos), the other to GPT.
MBR (Master boot record) has a different structure as GPT (GUID Partition table),
thats the part, where your hard drives are structured into parts.
GPT did come later, because it supports a partion greater then 2TB, what is the limit for MBR
Just not sure here, if you have to change also at the Bios settings to boot from legacy to UEFI, when you want to boot from the GPT disk, because legacy does not support GPT.
Basically, when you will boot your GPT disk ATA ST or ATA Samsung, which you have at a GPT partition table, you need to set also UEFI at the bios.
At your ATA CT drive, it may works with UEFI, if not, you need to switch back to legacy boot in Bios.
Next thing is your boot priority.
You can set up at your Bios the boot priority (del or F2 during boot), at which Harddisk should be looked for a OperationSystem or a boot loader. If you have multiple drives, you should have them all listed there, even USB.
You have to set there the right drive, where you want to boot from, like ATA Samsung or ATA ST2000 first.
With the F12 key, you have a quick boot menu, where you can temporary manually choose the drive, where you want to boot from. But this is only temporary, because your Laptop will load by standard your Bios settings, what you can save and need to adjust to your prefered Harddisk.
So, if you want have 2 or more systems, you may want to install grub into the drive, where you install your OS on it.
But remember, even, when you make at one Harddrive 2 partitions, you can only install 1 bootloader on that disk. And as we have learned, you can also not mix grub from Mint or from Manjaro, because they are different.
Grub ie will autodetect and reconfigure the list from your bootable systems with every grub-update what you manually do or the system does during an update, even when you have 2 Systems on the same disk in 2 Partitions.
- Partition1 (sda1 100GB Manjaro
- Partition2 (sda2 100GB Debian
Grub will be installed at /dev/sda and will show you Manjaro and Debian to boot.
When you have 2 Disks, you could install a bootloader at each disk, but then you need to change the boot priority at the Bios from driveA to driveB, that it loads the second grub installation.
Disk1 sda - Partition1 200GB Manjaro
Disk2 sdb - Partition1 500GB Debian
Grub can be installed once at sda or sdb, Bios settings needs to point either to driveA or driveB.
When grub works proper, it would show both times both systems to boot at the menu.
I am not sure at this point, if it would work, when you ie install a grub from Manjaro system into
ie /dev/sda, have Manjaro sys on /dev/sda1, got ie Nobara at /dev/sda2, and install Nobara grub into /dev/sda2, grub-update at sda with the Manjaro system, that it finds both.
But usually, if there is a system, it should not need such a ‘hidden grub’.
So seems you did only overwrite your manjaro grub with Nobara grub.
Fedora itself is some different from Arch or Debian and not really compatible. And not sure, how well her grub works.
But you also could skip a grub installation, and grup-update from an other system.
At this point, if you destroy your boot up, chroot becomes handy. You can log into the system you want, gain the rights to make installations like install your boot loader new.
For your system, its save, when you make a seperate partition or hard drive as your /home drive.
There you have all your personal datas, and the system gets installed at different partitions.
You only need to bind it manually to a new system during the installation process.
Then you can try what you want at the other partitions, but your personal datas are more save.
For F12, i use it only for quick boot up tests, to see, what bootloader is installed.
That way you can easy rotate through, what is installed at drives.
And mainly, when i install a new system, i usually always pick a second disk, where i can disconnet others, when i assume, something can be messed up
So, basically, you should now sort your boot order, install grub again on that priority1 drive, bind the home into your system, if an old one exists, and if you want install an other system, remember the letter from the new drive and install the bootloader from the new system on the same drive as the installation.
fdisk -l and lsblk gives you an easy overview all time, what letter is assinged to a drive or what partitions exist there.
If you figure out how to dd your harddrives back to you previous status, you only need to install the proper grub into your booting disk and, if debian would mess it up again, chroot into manjaro and grup-update /dev/sda.
I dont know too, if some systems has the routines during an update, that they auto-install grub into the first harddisk. I guess i had once the situation, and as i did try to update an second system, i did first change the boot priority to this system, like the Debian drive, because the routine seems did plainly grup-update /dev/sda, no matter where the Debian installation was.