FATAL ERROR: Unable to install GRUB in /dev/sda (dual boot; Ubuntu and W10 on separate drives); tried with and without EFI partition; my W10 is UEFI based

[Luke Goshen]

I have a Dell G3 gaming PC with 16GB RAM and 256GB SSD(W10) + 1TB HDD(where I want to install Ubuntu)
I have previously installed Ubuntu 18.04 on the same HDD (which I deleted later) without using a new EFI partition and the device for boot loader was set to the same HDD (and not the W10 HDD where Windows Boot Loader's EFI was already present)

Just so you know, I have fixed an issue that was coming up before this one - the installer was freezing at 'detecting file systems'; an online forum user had recommended I add acpi=off to the install command line - this solved this issue --if this affects any of the things that should/should'nt happen, for your judgement.

I have tried various combinations and almost all suggested fixes online and none of it will work -
with EFI, without EFI - all of it.
Please help as soon as possible.
Thank you in advance and god bless you!

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: ubiquity 20.04.15
ProcVersionSignature: Ubuntu 5.4.0-26.30-generic 5.4.30
Uname: Linux 5.4.0-26-generic x86_64
ApportVersion: 2.20.11-0ubuntu27
Architecture: amd64
CasperMD5CheckMismatches: ./pool/restricted/n/nvidia-graphics-drivers-390/libnvidia-decode-390_390.132-0ubuntu2_amd64.deb
CasperMD5CheckResult: skip
CasperVersion: 1.445
Date: Sat May 23 12:59:53 2020
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu-budgie.seed quiet splash acpi=off ---
LiveMediaBuild: Ubuntu-Budgie 20.04 LTS "Focal Fossa" - Release amd64 (20200423)
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.