Very slow transfer speed on SDXC card, iMac 2017

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Very slow transfer speed on SDXC card, iMac 2017

Hello! I just purchased two 256 GB Sandisk Ultra SDXC cards to use as secondary storage, thinking it would be simpler than using external HD. With cards specs rated at 150 MB/s, UHS 1, C10, U1, I thought, wow, those things will scream!

Well, I'm seeing around 20 MB transfer speed, confirmed by both the Activity Monitor app and the BlackMagic Disk Speed test app. Using encryption will further decrease the speed in the 15-16 MB/s range. This is less than half the speed of my previous, 12 years old USB 2 hard drive!

I'm missing something here?

This is on a 2017 iMac 21.5 on Ventura. Thanks a lot!

iMac 21.5″, macOS 13.6

Posted on Dec 2, 2023 9:47 AM

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Answer 1

Top-ranking reply

Servant of Cats

Level 8

37,199 points

Dec 2, 2023 3:49 PM in response to Deromax

I second the idea that it is not good to rely on memory cards for long-term storage.

As for why card read speeds are low, it's not just the card that matters, but the reader. If the card reader is a USB 2 model, or is internally connected to a USB 2 bus, that's going to limit things to USB 2 speed. If the card reader does not support UHS-I, then when you use a UHS-I card, the bus interface will operate in one of the older, slower modes that both the card and the reader support.

https://www.sdcard.org/developers/sd-standard-overview/bus-speed-default-speed-high-speed-uhs-sd-express/

Your iMac has USB 3.0 ports, so if you got an external USB 3.0 card reader that features UHS-I (or better) support, you might find that you could read your cards a whole lot faster using it.

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Other things that can slow down transfers are the speed of the drive to which you are copying the files, and Finder copying overhead (if there are a lot of small files). The overall speed of a transfer will reflect the weakest link.

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Answer 2

rkaufmann87

Level 10

148,546 points

Dec 2, 2023 10:07 AM in response to Deromax

Using SDXC cards for additional storage is not wise. It would be much smarter to buy either an external HD or SSD. For an external SSD I would recommend the https://eshop.macsales.com/shop/owc-envoy-pro-sx. Using SDXC cards is dangerous as they can fail easily and yes, they are slower than SSD storage. This would be a huge upgrade in your workflow. You may want to consider buying 2 of these and use one for your startup drive. Doing so would be a HUGE upgrade in performance for a 21.5" 2017 iMac. You can learn how to do this in How to setup an external SSD as your startup disk

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Answer 3

Deromax Author

Level 1

33 points

Dec 2, 2023 4:12 PM in response to Servant of Cats

I read somewhere that the card reader in iMacs are connected to an internal PCI bus. Only the MacBooks/MackBooks Pro uses USB card readers. The System information section on my Mac is reporting 2.5 GT/s link speed for the card reader. I don't know how fast it is, but it sounds like it's fast!

That's sad that a great Apple computer would use inferior tech that can be beat by 15$ external readers from Amazon. I have used Macs for nearly 30 years. They're great but you can always count on Apple to cut a corner somewhere! :(

Anyway, I'll get an external SSD and resell the SD cards!

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Answer 4

Deromax Author

Level 1

33 points

Dec 2, 2023 10:14 AM in response to rkaufmann87

Thanks for the info. I'll look into an external SSD then. I actually watched the Samsung T7 pricing lately and it went for a nice price during the BF sales. I should have got one!

I already have an internal SSD, along with 16 GB RAM. This computer is plenty fast for my light use. Save for the slow SD card speed! ;)

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Answer 5

rkaufmann87

Level 10

148,546 points

Dec 2, 2023 10:56 AM in response to Deromax

Is your machine a 100% internal SSD or does it have a Fusion Drive. If it has a 100% SSD then yup it's fine, if it is a Fusion Drive nope no such much.

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Answer 6

Deromax Author

Level 1

33 points

Dec 2, 2023 11:31 AM in response to rkaufmann87

Yes it's a full SSD. It may not be a factory item however, as I bought the iMac used.

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Very slow transfer speed on SDXC card, iMac 2017

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