Q:

Arch Linux Dual Boot Windows 10 2 SSD's?

Good day everyone, I'm trying to install Arch Linux based on the wiki guide but I'm a bit stuck on the dual boot part.
I have a 240G UV400 Kingston one single partition for windows 10 upgraded from windows 8, from what I know it should use GPT with UEFI since I have a 100mb EFI partition that I had to rebuild several times so far.
Second SSD a 120G A400 Kingston that i would like to use only for Linux.
My laptop is a Lenovo B50-70 I5 dual core 4 years old with a dedicated AMD video card 8GB RAM.
Based on the guides I found online some of them use a single hdd to dual boot Windows and Arch Linux ( not my case ) and in the most guides the install is single boot with it's own partition.
My questions

Should I create a EFI partition that I can mount in /mnt/boot/efi?
Should I use the existing partition from windows? 
What would be the proper steps to take to get the devil boot? 
Sometimes I managed in boot manager to see the second ssd but I get some grub errors as unknown file system prob because I did a wrong install.

I've seen a couple options on how to dual boot or install a boot loader but not really sure which one to take and also if is is possible I would love to keep my fast boot for windows 10.

A:

I've done the exact same thing you're talking about here... first let me recap what I'm assuming you're asking so you can tell if I'm going in the wrong direction:

You have a UEFI windows 10 booting off of one drive
You want to get Arch linux booting off the second drive
You are unsure how to get this working properly

Let's start. You've already got the windows 10 installed, which has created the UEFI partition. That UEFI partition can be used ans you /boot partition when you install archlinux.
Mount your archlinux partition to /mnt. If you have a home partition, create a home directory on /mnt and mount that partition there. Adjust XY to match your partitioning.
mkdir /mnt home
mount /dev/sdXY /mnt/home

Create a boot direction on /mnt and mount the EFI partition from windows there.
mkdir /mnt boot
mount /dev/sdXY /mnt/home

When you're sure everything is ready, run
pacstrap /mnt base

to install the base packages. Continue on installing archlinux as you would normally. 
If you're having issues with dual booting, I would do the following:

Check the bios after installing and make your Linux bootloader the default
If you have no option to make Linux default, install the efibootmgr package
Run efibootmgr with no flags to see the current boot order
Use efibootmgr -A to set the windows boot as inactive (probably 0000)
Use efibootmgr -a to set the Linux boot manager as active
Use efibootmgr -o to explicitly set the boot order. Set Linux to the first.

As a last note, I'd recommend looking into systemd-boot for UEFI booting. Systemd-boot simplifies a lot of problems I have run into setting up a boot manager with grub.
That said, if you are going to go with grub, there are a few additional steps you might need to take. I know that grub requires a bios/boot partition to be created when setting up grub on a GPT partition. If that partition is not there, you can have problems later on down the road. As Windows has already created the partition structure on your main drive, you won't be able to create this partition. (See the comment by Johan Myréen for additional clarification)
As I've not enough reputation to comment yet, I may have to reply to any questions with another answer.

