Fails to install grub on newly-partitioned dual boot system

[Pointedstick]

Describe the bug The Calamares installer fails to install grub on my disk which it has just created a new partition in.

64bit system, 1TB Samsung EVO SSD, MBR + EFI, two existing NTFS partitions (one small boot partition and one large data partition). The large partition has Windows 10 on it.

To Reproduce Steps to reproduce the behavior:

1. Select "Install alongside" and configure the installer to create a 300Gb partition for Neon
2. Perform the installation

Expected behavior Everything works

Screenshots and Logs At the very end, it fails to install grub:

[8BitSloth]

I can confirm. I have a Toshiba Satellite C55-B5100 and distros like Manjaro and Endeavor will fail if I let Calamares is allowed to set the partitions. If I do my own partitions I have near a 50/50 shot of the installer working. If I encrypt any of the volumes the installer will fall 100% of the time. Every failure will display OP's error.

EDIT: I would like to add that I believe this has something to do with Intel's garbage UEFI. It seems that somehow both EFI and Legacy BIOS are enabled at the same time. The only thing the EFI settings seem to do is tell the PC if it should boot using legecy or EFI extensions, but both are still active so the installer doesn't know what to do.

[ghost]

I can further confirm. ASUS Crosshair VII Hero X-470, Ryzen 7 2700, Corsair Dominator 3200, Sabrent NVMe 512 GB x2. OS ArcoLinux - tested several ISO's, latest and LTS kernels. Also failed to install Linux Mint, KDE Neon, EndeavoutOS.

EDITL: An ASUS laptop user running Arco, and one who only provided that he was attempting with btrfs, also have had this issue.