Did I set up RAID 1 correctly - why does it show full size?

Hi there folks, I followed the Disk Management instructions from this site: https://www.guidingtech.com/4-best-ways-to-configure-raid-1-on-windows-11/ to configure my RAID1 array of 2 x 20 TB Enterprise HDDs (internal) on my new Desktop Tower PC, Windows 11.

However, when I go to 'This PC' and click on the new volume D drive, it's showing a total of about 40TBs. Because it's mirrored drives, shouldn't I only see 20 TB of available space because of the redundancy?

I'm a little confused how this works and if I set it up correctly.

Please advise, thanks!

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Ok so, storage spaces isn't the same as raid. A mirrored storage spaces pool, is not raid1. It's very similar to it in that it's a mirrored set, but it's not the same thing. In a raid, because see, in storage spaces, you can have a mirrored set with 3 drives, and you'll actually be able to about one and a half times one drive of data in that pool. This is because in storage spaces, it's the DATA that is being duplicated, not the drives. So don't confuse the concepts.

Now, as for why it shows that size, well, because you configured it to. Storage spaces completely decouple the pool size from actual currently usable space. You can create a pool of only 1 8gb drive, and yet say it's a 1PB pool and it'll happily do it for you. You'll still only be able to actually store 8gb ofc, but the pool will report rhe 1PB of maximum space.
- Thanks for explaining Storage Spaces, I was wondering about this! However (sorry the link I sent is super confusing), I followed the instructions for Disk Management RAID creation (Number 3 down the page), not the Storage Spaces one.
  
  BUT I did at first tried the Storage Spaces pool but found the same problem, so I deleted the pool and removed the drives and started over with Disk Management. I'm not sure if that affected anything. The drives appeared like they did after I restarted - unallocated.
  
  Number 3 there is just... wrong. It shows selecting striped, bit striped is raid0, not 1, so if you selected that, that's why. If you selected mirrored though you should indeed only see 20tb if each drive is 20. There should be no issue with having had storage spaces previously no.