I'm wanting to build a file/Plex server with Linux Mint 64bit (latest version).

I have 3 drives:
- 1 x 250GB SSD (OS)
- 2 x 4TB Sata Seagate


I have created a RAID1 array in the BIOS with the 2 4TB drives. Next I would like to mount the RAID1 array to a fold (/RAID_Drives). I open a terminal and run "ls -l /dev". I see sda, sdb, sdc. Where is the RAID array I created in the BIOS?

Why am I seeing all the individual drives? In Windows if you RAID 2 drives you end up seeing 1 drive total in "My Computer".

First, where is the RAID1 array and second how do I mount it?
[mod removed redirection links]

Start here, have a look around. Note particularly the discussion of "fake raid".
You need to know what flavour you have, and whether it is supported.

Better - set up mdadm software RAID for all the reasons mentioned on that link.

A better way of listing all storage devices are the commands
Most likely, the Linux kernel does see your RAID1 device as a single disk. lsblk tells you the size of storage devices, which gives you a clue which is which. lsscsi gives you information about vendor and model.

I don’t know how Mint handles RAIDs, but on the command line, you make a file system on you RAID device, add a line to /etc/fstab and run mount -a.

Slightly off-topic: You should consider using a software RAID, specifically md RAID. Reason: Your BIOS-based RAID may be proprietary and not compatible with other computer, motherboard or BIOS models. When your computer breaks, you risk losing your data. MD RAID on the other hand is part of the Linux kernel.