First time NAS setup

i5 processor

What i5? If it has an iGPU you most likely should remove the 970. Will just needless eat power and PCIe lanes in this setup.

setup both hard drives as a raid 1

You mean RAIDz1?

I would consider something else than TrueNAS and ZFS as a home user. It is a robust OS and file system but usually not the best option for most small home setups. mergerFS+Snapraid or Unraid are far more flexible as they allow you to add single drives of any size down the line. The performance and reliability are usually good enough for a home user with a 1, or 2.5Gbe network.

Once your system is setup tinker a bit with the OS, setup some file shares and once you know how to set up everything properly copy your stuff over and verify afterward if it was done correctly. Not really a big deal.

There shouldn't be any problems with that. Here's basically your question: https://www.truenas.com/community/threads/transfer-data-from-usb-hdd-to-freenas-box.24226/

I'm mostly anxious about somehow losing all my data when I make this transition since I have zero experience when it comes to running a NAS ....

Not understanding how you will set up the vdev without losing data.

Here is my understanding of setting up Truenas.

You have that data sitting on a drive. At some point you will copy that data FROM the drive onto the VDEV.

But.... whatever drive is inside the NAS that you construct the VDEV from - to turn it into a VDEV you are going to need to (effectively) format the drive. You can't get around this. I am not aware you can take a drive with an active partition and data and preserve that partition and data.

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I have a similar but slightly different problem. I have a solution that sucks, but it works. You might be able to adapt some piece of it for your own problem.

I have a 300 tb Gdrive account. Google got wise to me, sent me an email to tell me - and I quote, 'Cut your shit out. Also, we are freezing write access. Have fun. Asshole.'.

(you sort of have to read between the lines)

I have a 40 TB folder that I am desperate to keep. I don't really have 40 tb of free space.

So I am building a NAS. In the meantime I am terrified of Google shutting my account down.

I took a 7tb drive on my computer and purchased a BackBlaze account and did the upgrade to the account where they keep deleted data for 1 year. 365 days.

I read 7tb of that 40tb into that drive then allow backblaze to upload that data. When it is done I delete all the data and do another 7tb.

I can go into the backblaze account and recover the deleted files for 365 days so... the clock is a ticking.

(if you pay attention to the scheme you will see I am after redundancy. If all goes as I think it will go I will populate that folder from the Google Drive account. If Gdrive goes poof (as I fear) I can download it from Backblaze)

Copy, don't move. That way, should it fail, your first drive is still accessable..no?

What filesystem does the original HDD use?