Yeah, decide on ONE system. I would recomend efi because that is what is almost assumed nowdays.
But if it works with old boot, maybe you want to stay on that, idk, only you know.
As I said before, I have limited knowledge of old boot (witch you correctly stated is the boot you are using) so I should probably leave this to others and learn from the sidelines.
Check if Manjaro is really booted in BIOS by checking this directory: ls /sys/firmware/efi/efivars If empty/not there, it is BIOS.
Create a 10MiB unformatted partition (preferably at the beginning of the disk), flag it with bios_grub and name it bios-grub (so you know later what that is). With this partition you can write the MBR on a GPT partition table, when running grub-install.
Then you would use Manjaro’s grub bootloader.
Hwoever I would strongly advice you to use EFI and only BIOS, if the OS doesn’t support it.
ls /sys/firmware/efi/efivars
ls: cannot access '/sys/firmware/efi/efivars': No such file or directory
I won’t be able to do this at the beginning of the disk. Do I have to do something differently if it is is not at the beginning? Can the partition be part of an extended partition?
I would like to stay with BIOS instead of EFI. I just don’t want to have more things to check. With BIOS I know that it works.
I just used the Pamac GUI and could easily auto update with both of my 2 systems running in MBR/Legacy mode… was a flawless update.
Just create timeshift snapshot and prepare a Manjaro USB Stick… just in case.
I personaly think that MBR is superior to this stupid UEFI features anyways… its a big downgrade related to security… applications or a virus can change your UEFI settings while the good old Bios is still protected, is a big plus to NOT use EFI.
I also remember a AMD driver issue, 1,5 years ago… where a AMD driver changed UEFI Voltage (on CPU’s or GPU’s, i can’t remember) settings and alot people had a unstable systems from it!!!
This is total DANGEROUS… dont use UEFI, as long as possible stay with BIOS/Legacy.
To bad that the new GPU Vendors forced us to use UEFI… instead the much better BIOS (with less dangerous FEATURES).
This disk (currently on /dev/sda) is the GPT partitioned drive, containing Windows and Linux. As Bedna mentioned, it’s likely that disk is already configured to boot as UEFI. You can possibly find the model number on the label or the end of the disk, to identify it visually.
ST1000LM014-1EJ1
The other disk (currently /dev/sdb) has Legacy/MBR partitioning.
SAMSUNG HM641JI
Here’s the issue: Booting 2 disks with different boot technologies will guarantee ongoing problems, in a multiboot environment.
You can choose UEFI or Legacy, but not both.
If you choose UEFI, then you need to disable CSM in the BIOS to boot UEFI exclusively, and sacrifice the Legacy disk and its contents (save any data you need to USB, or similar).
Then you would need to create a new GUID Partition Table (GPT) on that disk and prepare it for UEFI boot. If you boot the Manjaro installer and choose manual partitioning, you can do this (carefully) during install.
I also recommend disconnecting the current UEFI disk (ST1000LM014-1EJ1) before installing Manjaro, to avoid likely complication. You can reconnect it later when Manjaro is installed.
There are more steps involved which no doubt someone will help you with once you decide how you’d like to progress.
In the meantime, have a good Xmas. 
He will lose all data not only a partition on that disk, so he need to backup his whole harddrive from changing from MBR to GPT. This also include a full OS reinstall.
Edit: I just write that, to make 100% clear… how big this change, will be.
And it should be well planned.
Yes, as was inferred by the word sacrifice. 
I’ll come back to this when I have an opportunity, that is, if the OP isn’t further confused by differing opinions in the meantime.
Enjoy Xmas, if that’s something you do. Cheers.
So wie are talking about sda5 right? That is Manjaro?
Usually grub can find the partition which flagged with bios_grub. That one can also be at the end, but maybe the UEFI needs that.
GPT can have up 128 primary partitions. No need for LBA.
Well then stay with it and limit yourself.
I hear you, but note that even if you run in compatibility mode on a UEFI, it is still UEFI, only with a BIOS emulation. Its like starting a PSX emulator, instead of running real hardware.
Yeah he should limit himself… and archive the full experience with possible dangerous Voltage changes from Drivers? /sarcasm off
German AMD issue Topic:
To me it seems you are booting from the mbr on the B disk, even if the manjaro is on the gpt A disk. That is why there is no uefi esp on the A disk.
I do have a running Manjaro installation and don’t want to reinstall Manjaro.
I do not want to scarify any disk either. That is why I ask before I do something stupid.
I want to use BIOS not EFI.
Assuming I can create a partition like that (I cannot try right now because I wouldn’t have enough time for troubleshooting).
Would the next command write automatically to this partition?
It feels so dangerous running a command on an entire disk. What would that command do if it does not find a partition like mentioned above?
If that command worked and I did
grub-mkconfig -o /boot/grub/grub.cfg
Would this be sufficient? Or do I have to do something else? Would this do anything to the other disk?
Thanks for the help already.
Keep it calm. The whole topic you linked is 100% Windows related.
Yes it is… and? The Situation is still no differend, a application/driver/virus under OS (if its Linux or Windows, doesn’t matter) has FULL Access to your UEFI settings and this is my Point and is a true fact.
There are FEATURES in Hardware today, that other’s would call it bug or issue.
Happy Xmas everyone
Only one MBR can be written per disk. And Grub is smart enough to detect MSDOS and GPT. In case of GPT it will write to bios_grub partition, if not there, it will fail.
Anyway… if you don’t have time, then don’t change it. Reinstaling is needed when Manjaro’s grub manages booting.
The command above: sdy will be sdb in your case.
It’s your choice, of course; albeit the wrong choice, in my opinion. If your machine is UEFI-capable, and with the Windows installation already on a GPT partitioned drive, going fully UEFI seems like a no-brainer as is often said.
However, I won’t attempt to dissuade you; there are already too many competing opinions in the thread.
Good luck.
This topic has been created to address such doubts
Thanks a lot. This is helpful.
lsblk -o path,pttype,parttypename | grep -e 'dos' -e 'BIOS'
/dev/sdb dos
/dev/sdb1 dos HPFS/NTFS/exFAT
/dev/sdb2 dos HPFS/NTFS/exFAT
/dev/sdb3 dos W95 Ext'd (LBA)
/dev/sdb4 dos Compaq diagnostics
/dev/sdb5 dos HPFS/NTFS/exFAT
/dev/sdb6 dos Linux
/dev/sdb7 dos Linux s
This is the output of the command. If I interpret the results correctly I have an mbr system. So no need to have a new partition because it is written to the mbr?
I think I never changed something about grub when I put the hd in the laptop. I just changed the boot order. It is set to the samsung HD (sdb).
If that device /dev/sdb/ is the first in your firmware boot order then MBR would be in that device’s MBR.
Assuming the above is correct - this would be the command to sync the disk grub with the new version
grub-install --target=i386-pc --recheck /dev/sdb
Also see Install-grub: a new way to keep your EFI/MBR in-sync with grub package
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