Failed to install grub. It's a fatal error. I guess it's my fault. Nothing to do with installation process.

I restarted in the initial screen, while it's showing blank screen and a blinking cursor for few seconds, may be while it's still starting to boot the USB drive.

Failed to install grub. It's a fatal error. I guess it's my fault. Nothing to do with installation. I am installing it through a USB drive. In the BIOS I gave the boot target as the USB drive. On restart, it showed a blank screen with just a blinking cursor for few second. I hastily thought that the USB boot process would pick up next time and pressed Ctrl+Alt+Del. It booted to the installation process. But it showed just my hard disk drive /dev/sda not my partitions. There were windows, 2 logical drives, swap, boot along side 16.04 and 18.04 Ubuntu. Then I recovered the partitions using TestDisk on live USB drive. It's was then I realized that I messed the boot process while it's dealing with EFI perhaps. I might be wrong. I knew then that this installation won't be smooth, but I was happy that my existing files are safe. Still trying to figure out what I have to do to boot to hard drive successfully. This is my silly doing. This bug is unusual. Hope you won't waste precious development time on this bug. Thank you.

Also I gave /boot to a separate drive (500MB) and selected that drive to install boot manager in. May be this is the culprit, or It shouldn't be done this way. I'll try not assigning separate drive to /boot and instead leave it to be default. I attached the partition table I used.

ProblemType: Bug
DistroRelease: Ubuntu 19.10
Package: ubiquity 19.10.21
ProcVersionSignature: Ubuntu 5.3.0-18.19-generic 5.3.1
Uname: Linux 5.3.0-18-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu8
Architecture: amd64
CasperVersion: 1.427
Date: Wed Mar 4 00:06:24 2020
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed quiet splash ---
LiveMediaBuild: Ubuntu 19.10 "Eoan Ermine" - Release amd64 (20191017)
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)