Install Pop!_OS 20.04 LTS alongside Windows 10

[fitz.leo]

I have 128GB M.2 SSD and on the remaining HDD (which is 1 TB with 3 partitions - 1st with 500GB, second with 380GB and the third with 100GB) I want to install PopOS on the second partition. This HDD contained Windows installation so I formatted that partition. (note that I formatted only the C drive, now I use that partition for storage with the SSD, and the third partition is for storage, but for things I do not need that much but I will in the future)
I flashed the ISO on 16GB Flash drive using Etcher, but when I booted the USB, the installation couldn't finalize. It said that it failed due to a hardware error. I changed the boot priority on the HDD over the SSD (first to boot from the HDD) and the same error. I flashed the ISO again using Rufus and the same problem, again.
Both my SSD and HDD are GPT, but the BIOS is set up to Legacy mode. I changed it to UEFI but when I booted the Flash drive it said boot the kernel first so I disabled UEFI.
Then I deleted the remaining Healthy partition from the previous Windows 10, so that there were only 3 partitions left. I created only root partition for the PopOS and it failed because there were not BIOS boot partition..
Then, I created one partition and set it to bios_grub, and the installation finished! Normally it displays to restart or shut down the computer. I restarted it and booted directly in the Windows 10..... I restarted again and booted manually into the HDD, but again, the same problem, I booted into the Windows 10. So now, I don't know what to do.
Also I am using Lenovo V310 15isk.
I've been trying for the past 3/4 days and I'm tired, I don't know what to do, and I want very very much PopOS. Also, I am using Linux in school, but never installed one.
Any suggestions to help me?

 

[jglen490]

You have an M.2 SSD 128GB and a 1TB HDD. You want to use this for Linux and only Linux, if I understand correctly. Here's what I would do.

UEFI:
1) set to UEFI mode, no Legacy, no CSM
2) disable Secure boot
3) Set SATA to AHCI, make sure there is no Intel RST, no fake RAID

Drives:
Use the SSD for / and whatever swap mechanism (files or partition), formatted to ext4
Use the HDD for /home (to include setting up directories for specific storage needs

Liberal application of the KISS method. You can encrypt the disks, if you insist, but it's not necessary, unless you are doing super-secret squirrel stuff.

Sit back and enjoy your Linux, and make it work for you rather than the other way around.

 

[fitz.leo]

When I boot in UEFI mode it says yo load the kernel first

 

[fitz.leo]

On F2 this oppened

 

[jglen490]

You're not in UEFI, according to those pictures. Usually, UEFI (or BIOS on older machines) is obtained from a cold boot and immediate keypress of some machine specific key such as F2 or DEL, but before the UEFI/BIOS firmware passes off to the OS.