I'm still confused. Actually, I think I'm afraid of failing.
Now on my HD I have 8 partitions. Here is my scenario:
/dev/sda1 Swap
/dev/sda2 EFI system of old installations
/dev/sda3 Elementary
/dev/sda4 Partition only of files for backup
/dev/sda5 /boot - EFS system linked to KaOS
/dev/sda6 / - KaOS installed and I'm currently using
/dev/sda7 EFI system of old installations
/dev/sda8 Old Kaos installation with grub (broken and disabled)
My efibootmgr:
BootCurrent: 0002
Timeout: 0 seconds
BootOrder: 0002,0001,0005,0006,000B,0004,000C,000D,0003,0000
Boot0000* Windows Boot Manager
Boot0001* ubuntu
Boot0002* Systemd Boot Manager
Boot0003* linuxmint
Boot0004* Windows Boot Manager
Boot0005* Linux Boot Manager
Boot0006* Windows Boot Manager
Boot000B* TSSTcorp DVDWBD SN-406AB
Boot000C* ubuntu
Boot000D* ubuntu
and my efibootmgr -v
BootCurrent: 0002
Timeout: 0 seconds
BootOrder: 0002,0001,0005,0006,000B,0004,000C,000D,0003,0000
Boot0000* Windows Boot Manager VenHw(99e275e7-75a0-4b37-a2e6-c5385e6c00cb)WINDOWS.........x...B.C.D.O.B.J.E.C.T.=.{.9.d.e.a.8.6.2.c.-.5.c.d.d.-.4.e.7.0.-.a.c.c.1.-.f.3.2.b.3.4.4.d.4.7.9.5.}....................
Boot0001* ubuntu HD(2,GPT,8ce8772e-1231-4f74-ade5-e557237f4c90,0xd00000,0x100000)/File(\EFI\ubuntu\shimx64.efi)
Boot0002* Systemd Boot Manager HD(5,GPT,7691b24f-0269-430d-9678-19a290cb217c,0x41000000,0x100000)/File(\EFI\systemd\systemd-bootx64.efi)
Boot0003* linuxmint VenHw(99e275e7-75a0-4b37-a2e6-c5385e6c00cb)
Boot0004* Windows Boot Manager HD(2,GPT,8ce8772e-1231-4f74-ade5-e557237f4c90,0xd00000,0x100000)/File(\EFI\Microsoft\Boot\bootmgfw.efi)
Boot0005* Linux Boot Manager HD(2,GPT,8ce8772e-1231-4f74-ade5-e557237f4c90,0xd00000,0x100000)/File(\EFI\goofiboot\goofibootx64.efi)
Boot0006* Windows Boot Manager HD(7,GPT,4013bb95-7b3b-4c24-83f6-267c307fa803,0x66900000,0x100000)/File(\EFI\Microsoft\Boot\bootmgfw.efi)
Boot000B* TSSTcorp DVDWBD SN-406AB BBS(CDROM,,0x0)AMBO
Boot000C* ubuntu HD(2,GPT,8ce8772e-1231-4f74-ade5-e557237f4c90,0xd00000,0x100000)/File(\EFI\Ubuntu\grubx64.efi)
Boot000D* ubuntu HD(7,GPT,4013bb95-7b3b-4c24-83f6-267c307fa803,0x66900000,0x100000)/File(\EFI\Ubuntu\grubx64.efi)
I studied the efibootmgr commands a bit and tested start with all inputs (sudo efibootmgr -n 000x). I noticed it has a TSSTcorp DVDWBD input, which seems to be my DVD player. It did not start with live cd too, after I chose 000B. By analyzing the outputs of the commands, I can conclude that they are old plant residues that no longer exist in HD. From what I understand I can delete those entries and leave only the one that works, right?
I used to secure a UFI partition for each install, but I almost always chose to let Grub manage the boot. Then Elementary will not start that way. My only reason for having 2 systems working is that if something goes wrong in KaOS, I can start an OS on the same machine to try to recover it. That's what happened this week.
I learned a lot from this topic. Thanks for the help and encouragement. I'll get a solution, even if I have to take the HD to another machine.
KaOS is great!