iMac Internal SD Card Slot vs Thunderbolt/USB-C Reader

Hello, Geeks!

A quick question: does anybody smarter than me know whether the built-in SD card port on the iMac is faster than a USB-C reader plugged into one of the Thunderbolt 3 slots?

I just realised I could probably test this, but someone smarter will definitely know something interesting on this matter.

Thanks, Pals!


I think it would depend on the capabilities of the USB reader. I have seen many USB 3 devices which are still using USB 2 internal components. Perhaps this is less so with USB-C devices but an inspection of the card reader’s specs would be recommended.

To do a quick check of the SD cards readers I have on my iMac Pro, I ran a speed test using Blackagic Disk Speed Test. The SD card tested was a SanDisk Extreme PLUS (32GB, SDHC, 80MB/s read, UHS class 3); not the fastest card out there but the fastest one I had that I could find quickly.

The internal SD slot tested at 52.1 MB/s write and 90.7 MB/s read.

The same SD card in a Satechi Aluminum Type-C Clamp Hub Pro tested at 50.7 MB/s write and 76.6 MB/s read.

This Apple KB article says that internal SD card readers on desktops communicate over the PCIe bus so tend to be faster than USB readers such as those in Mac laptops and external readers.

Of course, SD card type, class, etc. will also have a factor in how fast data can be read from / written to the card. Cheap, slow, older cards will be slow no matter how fast the reader can operate.

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As @JoePreiser said, the card itself will probably be the bottleneck. I did some benchmarks on what was supposed to be a fast SD card, if it’s helpful. (Internal slot, 2015 MacBook Pro.)

Another consideration is the iMac SD card slot is in the back, so any speed gains would (for me) have to outweigh fishing around on the back of the iMac to plug in/remove the drive.

In my experience, it has been surprisingly difficult to line up:

• (part 1) a fast card with regard to a particular camera (or slot on a camera)
• (part 1) a “reliable” or “robust” card … e.g., at least three SD cards I’ve had have simply broken; one in camera (essentially ruining the camera, as the cost to fix it/extract it exceeded the camera’s value)
• (part 2) a fast USB interface between the card and the computer
• (part 2) an interface that includes both CF and SD slots
• (part 2) a short cable / compact enclosure

To the point of your question … “it depends.”

Hi… I would go with the Sandisk from Apple, since they have a 20% price reduction atm, it’s a reliable manufacturer and as Apple are selling it you can be pretty sure it will work well with their machines.
Since you also want to attach external disks (which I presume you have and are not USB-C) you might want to consider whether a multi-function adapter could better suit your needs, for example this. More expensive, but maybe less fuss. Note it says ‘coming soon’; there will be similar from other manufacturers.

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It’s a complicated matter.

As far as Apple goes you can read your machine card reeder speed quite esaily:

as stated here

You can usually find external readers specs on the manifacturer website (in it’s not a generic chinese brand, I guess): I use this one from Lexar connected to a thinderbolt 2 hub, and it is well suited for my needs.

Another factor is the specific card speed: Alik Griffin’s website is some sort of a card speed bible to me.

Then there is to factor in your specific needs: what do you need to transfer? Tens of jpeg photos o big video files?