Low write speed on Ultra 512Gb micro SD

Memory Cards SD | microSD | CFast | CFexpress | CompactFlash
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I bought a used SanDisk Ultra 512Gb micro SD on eBay. It writes at 7-8 Mb/s which I think is absolute rubbish. It reads at about 60 Mb/s.

I have other makes of card which give me write speeds of around 40-50 Mb/s. None are top of the range Extreme or Pro type cards.

Is my Ultra writing normally or is it a dud card?

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Hi @Alpaca

Have you checked our knowledge base articles?
(Get Support for Your SSDs and Flash Products | SanDisk)

Try this one.

Have you opened a Support Case?
If not, contact SanDisk Technical Support for assistance.
(Contact Support | SanDisk)

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Thank you. I bought it “Used” on eBay, so can I claim against SanDisk?

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"Hi @Alpaca ,

Since the card was bought used on eBay, it’s likely not covered by SanDisk’s warranty. The slow write speed might mean the card is damaged or fake. You can check if it’s real using SanDisk’s Memory Zone app or similar tools. If it’s fake or faulty, try contacting the eBay seller or opening a dispute. Let me know if you need help!"

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Hi Helly. Thank you for your reply.

The app SD Insight says it’s a SanDisk made in July 2024.

Alan’s True SD Capacity/Speed shows a write speed around 7Mb/s. During the write test the speed fluctuates constantly, quite unlike my known good cards of other makes. Real life file transfer shows a write speed starting just in double figures but slowing to about 7Mb/s. This is rubbish.

Read speed is around 60Mb/s with zero errors. Capacity is over 300Gb and counting so not a smaller card.

So I think it’s a genuine SanDisk. I have told the eBay seller and he says the Ultra is nowhere near as fast as some other makes, and this is normal.

Of the 512Gb micro SD cards in the lower price range, i.e. not the top priced heavy duty “Pro” type cards, the SanDisk Ultra is the dearest. Cheaper cards write at 40-60 Mb/s when I use them. I cannot believe that the more expensive SanDisk Ultra is made to run at only 15% of its competitors’ speed.

Is this performance normal for a 2024 Ultra, or have I got a dud? Comments on earlier threads refer to much older cards.

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Thank you. I bought it used on eBay. I use it in my Android tablet for which it is correctly formatted to exFAT.

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The card you are describing has 10-20 MBps for larger capacities like 512GB and a read speed of up to 100 MBps. But a write speed of 7-8 Mbps for a 512GB card is unusually slow, even for this model.
Since you bought it used on eBay, it’s worth verifying the card isn’t false. False cards often have incorrect capacities and have abnormally low write speeds. You can use tools like H2testw (Windows) or F3 (Mac/Linux) to test the card’s actual capacity and performance. Try to format it and see if you can regain the full capacity of the card.

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I know this is an older thread but I thought I’d add some insight to this thread. I have one of these that I bought new probably about the same time (2024) and it too has a slow write speed. I have always generally regarded these Ultra cards to be on the slower side, with the Ultra Plus, Extreme and Extreme Pro cards to be quite a bit faster (the Extreme Pro cards being the fastest). Keep in mind the speed rating of the Ultra cards which is usually V30 / A1, which is pretty slow by today’s standards. Mine currently is writing between 7-10 MB/s which seems to be “normal” for this class of card. They are meant as storage cards and the compromise to get large capacities, at lower prices, they have chosen to give it a slower controller or reduce it’s speed. If you want more speed, you need to get an Extreme Plus card (which is what I’d say is one step down from an Extreme Pro, but a few steps up from the basic Ultra card you have).

Second, I would not buy used memory cards from eBay (or even new ones). Counterfeit cards (even ones “sealed, new in the box”) can be counterfeit, so I would only buy from a reputable sources and eBay (and in many cases Amazon) sellers are not a trusted source. Buy either directly from Sandisk (or the manufacturer) or a well known supplier like BestBuy, B&H Photo, Adorama or a few others. Just because a piece of software can detect the controller and say it’s “Sandisk” doesn’t mean much in today’s computing world is people can flash controllers on cards and SSDs to say whatever they want really, and masqurade as a legit card when in fact it’s not, and one red flag is that the fake cards are unusually slow (although in our case, this is just how these cards are, but if you buy an SSD and it’s extremely slow, it could be counterfeit if you didn’t buy it from a reputable source). So where you buy stuff from does matter when it comes to memory cards and storage media. To perform tests either h2TestW for Windows works well (although I think it’s read/write speeds are not entirely accurate), or use another tool like CrystalDiskInfo (which is popular with SSD users) to check drives and verify write/read speeds. Keep in mind most manufacturers only advertise the read speed, which is always faster and some otherwise “slow” writing cards will read very fast (say write 10MB/s but be able to read up to 120MB/s for example).