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Introduction

I have a microSD card with data that I'm trying to connect to my computer. My computer has slots for connecting to a USB or an SD-card, but not microSD. (example below)

However, I have two microSD adaptors that can connect to my computer as either USB or as an SD card. (example below)

Question

Will either adaptor transfer data faster than the other? Are there any advantages of using one over the other?

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  • There is considerable variation in speeds of flash devices, whether USB, SD or µSD. Also, "speed" needs to be defined: random-access or sequential, read or write speed. This information is readily available on the web, cameramemoryspeed.com/reviews/micro-sd-cards and cameramemoryspeed.com/reviews/sd-cards, for example, and there seems to be no systematic difference between SD and micro cards.
    – DrMoishe Pippik
    Commented Jun 12, 2022 at 3:41
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    MicroSD and SD cards are electrically identical, it's just a form factor change. No sources are needed beyond the specifications for the respective cards. Most cards these days seem to be microSD cards that come with an SD apdater; most actual SD cards are probably slower because they are older and have slower specs, not because they are SD. The adapter has no electronics in it. USB can't speed up a card, because it is still using the same bus between the card and the USB adapter.
    – user10489
    Commented Jun 12, 2022 at 5:07
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    There is one difference, however. Most of the SD adapters are super cheap and not designed for very many insertions. The contacts in the adapter will wear out quickly if you pull the microSD in and out frequently. (But if you've got spares, who cares...)
    – user10489
    Commented Jun 12, 2022 at 5:12
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    Actually, the flash memory in the card is probably the limiting speed factor, not the bus. But adding another bus (usb) isn't going to change the speed (unless it makes it slower).
    – user10489
    Commented Jun 12, 2022 at 5:13
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    In retrospect, it isn't that simple. The SD/MicroSD standard actually supports multiple bus protocols. Normally the fastest protocol available is negotiated. But in some degenerate situations (like, a really old SD to USB adapter), it might be possible that a slower bus protocol is used.
    – user10489
    Commented Jun 12, 2022 at 5:19

3 Answers 3

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My take is that theoretically there shouldn't be any difference which adapter you choose - in both cases the speed will be limited by that of the microSD. In practice, small differences may be caused by the quality of the chosen adapter and the computer port to which you plug it in.

An SD adapter is just a passive wiring adapter with no active components, so it shouldn't have any impact on the speed. The USB interface by itself is much faster, but this doesn't help the adapter go any faster than the speed of your microSD card.

Answer : Use whichever adapter you prefer - the speed differences, if any, will depend principally on the quality of the adapter that you have bought. If both adapters are of good quality and support the highest speed of which the microSD card is capable, then the speed difference on the same port will be negligible.

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To understand differences in performance in these two cases, you need to consider the hardware involved and the corresponding data path.

To function, a SD (or microSD) card must have a SD controller. SD is a fairly complex standard, and the controller must perform several actions to identify the card capabilities (one-lane vs four-lanes, or even basic SPI) and determine the speed of supported interface. Then switch the data lanes into proper mode (if it supports them).

Now we have:

  1. USB case. You have an internal PCIe bus > [host USB controller] - > USB port > USB "adapter". The USB adapter has a USB-to SD controller bridge, and a SD controller1. Depending on when it was produced, the SD controller may support all latest SD speed modes, or may not.

  2. The built-in SD slot uses most likely something like Realtek RTS5208 controller (like my old laptop has), which resides directly on external PCIe bus. This is a different SD controller, lets call it controller2.

It might seem obvious that the Controller2, being directly connected to the internal PCIe bus should perform better, but it is not for sure. There could be difference in performance in all directions.

First, the SD_Controller1 is different from your SD_Controller2. One may support all speed modes for your particular micro-SD card, the other may stay in older slow modes (since every SD is backward compatible). So the bulk performance will be seriously different.

Second, the USB host is connected to the system via internal 16-lane architecture, which could be running at Gen4 speed. The embedded Realtek likely uses 1x PCIe lane, maybe at old Gen2 or Gen1 speed. This could be a bottleneck, which would favor USB adapter.

Therefore, the performance difference can by anything, all depends on particular microSD capability, SD controller capability, and bus interface throughput.

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Sandisk Mobilemate UBS 3.0 Micro SD card reader is 2 times faster than regular SD card slot.

enter image description here

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