Is my data redundant?

[TribalHound]

In layman's terms a storage pool is like a pie, and they can vary in size.
A volume is a portion of that pie.
You can cut the pie into variable sized slices, or you might be hungry and eat the whole pie in one large portion. If you invite lots of people for dinner (multiple users) you’re going to need more pies.

Since your NAS has 2 x 4TB drives the total usable space as a single pool with no redundancy is 8TB.
If you created a RAID1 arrangement you would only have 4TB of usable space. RAID1 is where your NAS automatically clones all your data to both discs. Then, if a single disc fails you follow the NAS instructions, swap the failed disc for an empty one (same size or larger) and let the NAS take care of re-cloning all your data.

Raid is NOT a backup, it’s just insurance against dying discs & all discs die eventually.
All discs inside the NAS are managed by the NAS so if the NAS fails, is stolen or destroyed, you have no backup. So decide what data is crucial and either back it up to a cloud storage or get a WD my passport (or similar) and back it up via usb or to another hdd inside your PC.

If you’re using both discs without RAID, you can’t create it without reformatting, so you’ll need to move your data off first.

Within your system you’ll have Disc management where you can configure RAID, storage pools and volumes.