The details of why the HD needs replacement can be found in my 1st thread HERE.

I read an Article about a month ago that said that HGST HD's have been statistically proven to be the most reliable HDs made, and so when I found a

HGST Ultrastar 7K4000 HUS724040ALE640 (0F14683) 4TB 7200 RPM 64MB Cache SATA 6.0Gb/s 3.5" Internal Enterprise Hard Drive Bare Drive

on sale at NewEgg for $133.00, I thought I found a cheap and easy solution, but the more I researched, and the more I learned, the less "optimal" this HD seemed to be. The label says it was made in June of 2012, and one of the reviews alleges that these HDs were subjected to hard use for years, and then were somehow "zeroed" to appear like they are brand new. They are being sold by NewEgg as brand new.

Prices vary wildly, and a lot of stuff and fluff about what the purpose of the HD is, etc... (I don't believe any of this. I think it's all marketing.)

And because I'm doing lots of research I have a lot of questions about technical characteristics and whether or not they are worth extra money, or not.
Example, for some reason 7,200 rpm seems "right", 5,400 rpm seems "wrong" and 10,000 seems not worth the extra expense, plus it also seems like they would be more prone to failure.
I don't think the write cache matters, since this new HD is going to be one of two in a RAID 1 (mirror) array and I don't care about write or read times since the only purpose it will service is data storage. Note: I will not be running and software from the RAID array, ever.

I care about warranty, and I'm pretty sure I want "NEW" and not refurbed, used or open box. I think SATA II for some reason (vs. SATA I or III), and I think $150 is a reasonable price, because I've seen what appears to be comparable HD's at twice that price.

HGST is supposedly out of business, but somehow was purchased by WD and are still the same old HGST quality despite the WD label (and I'm cynical about this).
Other than that, I have no sense of what brands to either like or dislike. Some of the "Dell" HD's are quite cheap, but no clue what quality they might be.
In terms of budget, I figure I can get a "good enough" replacement for $150.00, but if the technical spec is worth the additional cost, I can pay twice that.

Any help appreciated, and thanks in advance.

Unfortunately, if you're looking for a hard drive with a genuine warranty, you'll have to buy from official suppliers what manufacturers are currently producing, and at the prices they're selling it at - apparently $300 and up is the current market price; you won't find anything cheaper. Of course, you can save on the disk class and buy not an enterprise-class, but consumer-grade junk, but one that comes with a 3-year warranty, and preferably not a SMR hard drive. But in any case, it looks like nothing new with a warranty is sold for $150 or less anymore. Since we're talking about a RAID array, due to fault tolerance, you don't have to look for the best or most reliable drive available. You can buy cheap consumer junk like a WD Red or a Seagate Ironwolf, and simply replace them under warranty when they fail.

There's no choice between SATA 1, 2, or 3; they're all compatible. The only difference is speed. A higher version means higher speed. A lower version means an older drive. All new drives support SATA 3. Any device with a SATA port, even a 30-year-old computer with an IDE-to-SATA adapter, can work with a hard drive that leaves the factory today.
This story ended in the last decade, and for consumers, it was around the time of the release of Windows 7.

5400 or 7200 RPM were very significant when Windows booted from hard drives, and a 64 GB SSD was prohibitively expensive. 10000 is a completely different story from the server world; there were no such drives in the consumer segment, excluding the WD Raptor, and they had a much smaller capacity than mainstream hard drives, they made as much noise as a fighter jet and heated up like a volcano. Nowadays, none of this matters much. 5400 RPM hard drives are considered somewhat more reliable precisely because of their lower RPM. Their read and write speeds are lower, though not significantly so. The same is true for the much-hated SMR technology: for a hard drive that's written tens of gigabytes a week, and most often only reads data, all the shortcomings of these drives are completely irrelevant.

What's a "genuine" warranty, vs. a fake one? What's a "genuine" supplier? I mentioned NewEgg. Is NewEgg "genuine"?

Here's an entire page of New, In Stock, 4 Tbyte, SATA 6 Gb/s, 7200 rpm, 3.5" Hard Drives ranging in price from $133.00 to $269.00, plus more expensive options on the 2nd page.

https://www.newegg.com/p/pl?N=100167523 ... 31&Order=1

Which one should I buy, and why?

One that allows you to replace the hard drive if it fails at any point during the warranty period - not just a piece of paper or empty promises from a seller who might vanish tomorrow.

I'm not in the US and can't tell you the right places to buy, but apparently Newegg is a fairly well-known and trusted seller, and it's highly likely that they get their hard drives officially from the manufacturers and provide a genuine warranty. But make sure you buy the hard drive directly from Newegg, and not from third-party sellers who might be selling products on their site.

Unfortunately, I can't tell you exactly what to buy there, because their site isn't working properly for me, and it won't open at all when I use a VPN.

NewEgg operates in America and Canada but not sure about other jurisdictions

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USB 3.5" enclosures are $19 USD postage paid from China, I own several