Why are your SD card transfers slow?

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Some varieties of SD cards.

There are a number of reasons why your SD card is slow when transferring to your computer, in 2021 most of them probably have nothing to do with your SD card being slow, but, of course that is also a possibility so lets start with that.

#1 Your SD card is actually slow

SD cards have been around awhile and because of that there is every chance you dug up an old card somewhere that is older and slower than any of your other cards. Your best bet with those old cards is to cut them up and put them in the garbage can. There is no getting the data on and off of them any faster.

The original SD cards don’t have any extra naming on them, it’s just “SD”. SD cards have been introduced according to size class, below are the different size classes currently offered:

Max Capacity2GB32GB2TB128TB
Bus Speed 12.5 MB/s25-3938 MB/s50-3938 MB/s50 – 3938 MB/s
Date1999200620092018

If you are thinking this chart does not help much, you are right. While it does give us a basic idea of what to expect the speed ranges are still gigantic and unrealistically high. No SD cards get anywhere near the maximums provided by the bus speed. This is one of the confusing aspects of SD cards that I hope I can clear up for you.

The next thing to know is the bus speed class of your card. So, in addition to the size classes listed above, there are also speed classes which are as follows:

Bus InterfaceBus LogoBus SpeedSDSDHCSDXCSDUC
Default Speed12.5 MB/syesyesyesyes
High Speed25 MB/syesyesyesyes
UHS-I50-104 MB/snoyesyesyes
UHS-II156-312 MB/snoyesyesyes
UHS-III312-624 MB/snoyesyesyes
SD EXPRESS985-3983 MB/snoyesyesyes

I bet you are thinking to yourself, wow, this is getting confusing! Well, we’re not done yet because these performance numbers are all solely about the BUS speed. The bus is the interface that is used to connect the card to the camera or the computer, it is NOT the card speed! In fact, all these numbers are purely theoretical and have nothing to do with the actual card speed. Which is why as you will note in the right hand column, all the size classes support all the different bus speeds. But, do you think you are going to get an SDHC card that can transfer at 3,983 MB/s?? Not gonna happen. Remember, these classes are JUST the bus speed NOT the actual card speed! So, while it may seem great to buy a UHS-III enabled SD card most likely the card will not reach those stated maximums.

In addition to the bus speed class there is the one and only speed class that actually pertains to the card itself and that is what the SD card people call the “Video Speed Class”. That chart is below:

Minimum Sequential WriteMax Bitrate (Video Quality)Video Speed ClassSpeed Class Logo
6 MB/s45 Mbit/sClass 6
10 MB/s75 Mbit/sClass 10
30 MB/s220 Mbit/sClass 30
60 MB/s460 Mbit/sClass 60
90 MB/s700 Mbit/sClass 90

There are actually other speed classes but they are so pathetically slow if you do not see one of the above logos on your card it is probably trash but don’t take my word for it try testing it and see. Or google reviews of your specific card, that might help you get a better idea.

If you are confused as to why these write speeds are so much lower than the BUS speeds from the previous charts you are not alone. Basically what becomes apparent is that SD cards are kind of a joke. They just aren’t very fast and the BUS speed is more of a pipe dream than a reality. That said a V90 class SD card should handle just about all forms of 4k including 4k RAW from most consumer and semi-professional cameras.

If you are confused and just want to “make sure” buying the V90 class card is a safe bet even though from what I have seen they’re pretty expensive.

That just about wraps up the card issues, so we’re on to the next possible reason your SD cards might be transferring slow.

#2 you’re using an old USB port or an old USB hub

Assuming you have determined that you have a relatively fast SD card to work with we can now move on to the next possible issue, your USB port or hub. From 1996 to 2014 there were 4 speed classes of USB introduced and they all used the same familiar rectangular USB plug known as a USB A connector. USB 3.2 introduced in 2017 uses only the new USB C type port. However, just having USB C does NOT guarantee USB 3.2 speeds! USB C is backwards compatible all the way to USB 2.0 so a USB C device could support any of the USB specifications from 2.0 all the way to the upcoming USB4… To try to state that as clearly as possible, USB 3.2 and USB4 ONLY support USB C, but USB C supports ALL USB versions going back to USB 2.0. Now, depending on the age of your computer you will probably be stuck with a USB port that is pretty slow. Here is a chart to help you make sense of that:

VersionUSB 1.0USB 1.1USB 2.0USB 3.0USB 3.1USB 3.2
Date Introduced199619982001201120142017
Max Data Rate1.5 MB/s1.5 MB/s60 MB/s625 MB/s1,250 MB/s2,500 MB/s

With USB 2.0 things are further confused by the fact that there were different speeds of connections possible with 60MB/s being the maximum but slower connections being very common due to device limitations. The point being that your USB 2.0 device could be running at USB 1.0 speeds depending on whether the device, such as your card reader or USB hub, actually supports USB 2.0 High Speed.

Additionally, there are actually two types of USB 3.1. USB 3.1 gen 1, and USB 3.1 gen 2. USB 3.1 gen 1 runs at the speeds shown above in the chart, but USB 3.1 gen 2 is a USB 3.2 speed protocol for USB 3.1 connectors. For instance, if you buy a USB A to USB C cable that says it is USB 3.1 gen 2 it will give you the USB 3.2 speeds.

Using the above chart you can see that any and all computers made before 2011 had USB 2.0 at best, so if your laptop or desktop is from 2011 or earlier this could be your reason for slow transfers.

The somewhat annoying thing is that even on my expensive 2020 desktop motherboard there are still some USB 2.0 ports! That is because the companies that make these things cheap out as many devices like a mouse and keyboard don’t need anything better than a USB 2.0 port. So even though you might have a newer laptop a common theme I have seen is that a laptop might have 3 USB ports on it but 2 of them will be USB 2.0 and only 1 of them will be the newest USB 3.1 or USB 3.2 or whatever. If you are confused which one is which you can check your laptop case for a text label, try looking at the side or bottom of the laptop near the USB port in question, or you can also check your computer’s online user manual. Just search for it on Google.

Another common issue is that if you use an old USB 2.0 hub with a new USB 3.1 port, you will only get the USB 2.0 speeds. So, if you are using a hub and getting slow speeds, try plugging your card reader directly into your computer and see if that improves things at all.

Of course the opposite is also true. If you plug a brand spanking new USB 3.2 hub into a USB 2.0 port, again, you only get the USB 2.0 speeds.

If you want the full speed of the USB port the card, the card reader, the cable, the USB hub and the USB port on your computer all have to support the same maximum speeds.

Speaking of cables, this is yet another tricky situation with USB devices. First of all many companies are out there selling non-standard cables, so watch out. As I said earlier USB C is backwards compatible with USB 2.0 and newer so just buying a USB C cable is no guarantee you will get the high speed transfers. You have to make sure that the USB cable you buy specifically supports the speeds you are trying to achieve.

That just about covers the possibilities with USB devices and cables so on to the next possibility!

#3 Your card reader is slow

A year or two ago I bought a SanDisk card reader to use with my SanDisk SD cards. I wrongly assumed that the card reader would support the high speeds shown on my card. The card reader only reads at a maximum of 70MB/s no matter what port it is plugged into. This despite my SanDisk card saying it could support 150MB/s reads.

As with everything you have to double check that your card reader supports the USB standard you are using, and it has to support the card standard you are using as well.

Ultimately I ended up buying a camera (the Canon R5) that supported CFExpress cards so I ditched the SD cards and stepped up to CFExpress, much to my chagrin I encountered issues with the first card reader I purchased and I ended up buying a Sony CFExpress card reader that gave me the maximum USB 3.0 transfer speeds of around 625MB/s.

I’ll say it yet again, when it comes to transferring to your computer every product in the chain must support the maximum speeds you are trying to achieve. It is counterintuitive that a card reader could support your super high speed CFExpress card but not support the actual USB spec that would enable those high speed transfers to your computer, but that is the case with many of these card readers. The situation will probably improve as time goes on but as of writing this post finding a card reader the will give you those full USB 3.2 speeds is a bit difficult.

#4 Your computer HDD is slow

At this point you’re probably thinking to yourself “now I know why people just give up and buy a Mac”. That is because there are literally dozens of things you have to know about how computers work just to transfer data from a card to a computer HDD. And my friends, the people who make these things think this all makes sense and is actually easy…

Even if you have the fastest USB, the fastest USB cables, the fastest USB card reader, and the fastest CFExpress cards in the world, when all that data gets downloaded onto your computer it still has to be saved onto the HDD or SSD and if the storage device in your computer is slow guess what?? Your transfers will still be slow. The fact is that while a CFExpress card can manage about 1,700 MB/s read speeds and the USB 3.2 spec can support that, the fastest mechanical hard drives will generally write to disk at about 170 MB/s, or about 10 times slower than the fastest cards.

In order to fix that problem you have to get a faster SSD in your computer. Typically that is going to be an NVME SSD. NVME is a specialized port for SSD’s that connects directly to the computer’s PCIE bus. Basically that means it has a lot of bandwidth and can support the fastest SSD drive speeds of over 5,000 MB/s.

If you have an older laptop you may be limited to SATA 3, that interface is limited to 600 MB/s which is still fast enough for most transfers you might encounter. But, if you are trying to transfer terabytes of data from your cards 600 MB/s may not be fast enough. 600 MB/s works out to about 32 minutes to transfer 1TB if that helps you decide whether you need to buy a new NVME enabled laptop or desktop.

IF you have a desktop computer with free PCIE slots you can purchase an NVME expansion card for your computer. This is one of the great things about building a PC, the PCIE bus lets you add functionality to your desktop for a relative pittance especially when compared to the cost of a new laptop!

#5 Transfers are still slow even though you know all of the above should be correct

In the case that you have verified all of the above should be correct to enable high speed transfers to your computer, there are rare situations where your drivers are incorrect. Usually Windows will automatically install the correct drivers but sometimes things get messed up. You might try reinstalling your operating system. And if that does not work, there may be a hardware failure of some sort. A cable may be failing or a USB port may be failing on your computer. In those cases try another port, USB hub, or even another computer or a friend’s or work computer and see if you get different results.

I learned from experience that many of these USB related products have questionable performance characteristics either due to buggy chipsets or just being sold as something they are not. So don’t automatically take your cable and USB hub’s performance claims 100% at face value. This is especially true if you went cheap and got one of those weird Amazon brands for a lot less money than seems normal.

If all else fails, call an expert and see if they can help you figure out where the issue may be.

#6 SD cards actually aren’t fast enough for your needs

The highest read speed for SD cards as of writing is 300MB/s, so if that is not fast enough for you there are really only two other options. Either get a camera that uses CFExpress cards or use an external recording device that uses CFExpress or SSD drives to save your video and photo data.

Conclusion

Phew, that was a much longer explanation than I was expecting. It turns out there are a lot of pitfalls in getting this stuff to work properly. End of the day you need to understand what you are buying and what is required to make it work as advertised.

If you have any questions or advice for others please feel free to leave a comment!

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