SSD860 is UEFI booting
also SSD840 because micro$oft/window$ has added entry in your /boot/efi
you can convert MBR to GPT on windows only
be careful , option in your EFI will be changed
( secure boot , fast boot , csm , legacy , AHCI , etc )
Thanks,
Did a quick google on that, seems rather complicated, and I could really screw up my Manjaro doing it
And really don’t know enougth to try that… in Windows or Linux
This usually happens when copying commands and not verifying the syntax afterwards in terminal. I would bet on that the original “--” had been replaced with “—” when you entered the command. 
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No, it looks good to me, copied that from my terminal
sudo grub-install --recheck --efi-directory="/boot/efi" --target=“x86_64-efi” --modules=“part_gpt part_msdos zstd” /dev/sda1
If there’s an mbr and gpt in my efi, maybe that why I’m getting this message : grub-install: error: More than one install device
Is there no easy way just to re write all the efi as that waould wipe the mbr in it, or am I dreaming or complety of track ?