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I bought a 256 GB SD Card from AliExpress for around $28 and at first assumed it was junk but the reviews seemed to be good.

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I used f3write and f3read to verify it does contain 256GB, and showed an average speed of 60 MB/s.

The result from black disk magic show:

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I am wondering why it was priced so cheaply and what other tests I might do? Could it be it supports none of the other formats? Is there any way it could get around f3write? Generally, what gives here?

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    Just thinking: If a cheap copy provides anything a brand product does - what makes it a "cheap copy", except the price? You got yourself something from a not-so-big manufacturer from one of the cheapest marketplaces out there and wonder what could be wrong? :) - reliability might not be equal to others, transfer speed seems lower than the ones i know from bigger brands, products like this might not really be certified or using defined standards, some say they could come with pre-installed malware... lot of things are possible, including that you got a working product!
    – xph
    Jan 24 '20 at 8:09
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    In the UK you can get an Integral brand 256GB micro SD for £28, which is about $33. $28 doesn't seem that wrong for an even more off name product that could still be legitimate. Even Sandisk aren't that much more expensive at that price point at the moment. Chances are you just got a cheap bulk no-name but legitimate product, they do exist. It's the 1TB cards at $50, about 1/10th the price of a legitimate card, that would be most likely to be fake.
    – Mokubai ♦
    Jan 24 '20 at 8:21
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    It is most likely legit. I have purchased 256gb flash drive for $35 almost 2 years ago - works without any issues
    – mvp
    Jan 24 '20 at 10:19
  • I purchased a 512 GB for less than $30 from Amazon. Only reason it would be fake is due to the brand being generic and the seller quality controls from AliExpress. Even if it’s real you only have a generic product so it’s performance likely is extremely poor
    – Ramhound
    Jan 24 '20 at 12:40
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Just because a program says that a SD card has 256GB it doesn't mean that it has all that storage. Flash drives/MicroSD card can have its firmware modified so it shows a capacity that is not real. You can check on Amazon that 256GB MicroSD cards are at similar prices from reputable brands as what you bought.

Looking through some reviews, some people did this test below and no issues there. You might have got a legitimate card with a slightly lower price in cost for reliability, warranty, etc.

In order to test a flash drive I use the program H2testw that:

H2testw works by writing large 1GB files to the free space and then reading them back. If the read data isn’t the same as what was written, there’s a possible issue. Source.

This test can take a few hours to complete depending on what size you wanna test. I would suggest that a 32GB test would be good enough to check if there's any issue.

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