As I am finishing my university course and will get graduated this week, I will clean my computers.
I decide not fixing or not reinstalling my Manjaro on external HDD on my Mac mini because I had always this problem. The grub got broken more than 10 times. You can see many topics of mine showing that I had problems with broken grubs after each update or upgrade.
I believe the issue is always the external HDD on an Apple computer, because each update or each upgrade, the grub gets broken if the HDD is external and the computer is a Mac due to blessing problems as @stephane has alerted. I do not know how to bless, preventing the grub of getting broken.
I love updating and upgrading, so I opt for rolling release. I am thinking of installing Linux on an internal HDD of my Mac computer, but I am not sure that it will happen the same thing after the next update or upgrade.
Someone said me it would never happen if the HDD on a computer was internal. My laptop is the example and it has never happened as the new SSD is internal.
If Linux installed on an internal HDD of my Mac computer gets grub broken after next update, I suspect it is Apple. Then I can conclude Apple is extremely closed-source as the EFI is. Very restricted. Then I will migrate to a new PC with open source processor, motherboard and video card the next year. In spite of buying a new PC, I depend on macOS because of three apps I use more: Affinity Designer, Sketch and Glyphs.
The final question:
Is it worthy to install Linux as the only OS on the internal HDD of my Mac computer or will I have the same issue? Or better to reinstall on external and reconfigure the blessing via terminal, protecting the grub from getting broken?