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I suggest those that think their SD slot is fine, test it more throughly with all your SD cards.

I'm the exact opposite of an expert on this, but I believe this is exactly the crux of the compatibility problem.

I don't yet have the 2021 MPB but I've had problems reading some SD cards with my 2015 MBP that read just fine with a USB connected SD reader. The issue is that the SD "standard" is anything but. It's something that grew out of previous efforts.

See this Wiki page https://en.wikipedia.org/wiki/MultiMediaCard for an older version of SD, just slightly mechanically different. If SD slots attempt to accommodate all these subtly different cards they can have slight mechanical differences that keep some cards from working.

Here is a picture of just one of the slight differences: https://en.wikipedia.org/wiki/File:MMC_card_vs_SDHC_card_undersides_angle.jpg
 
Transferred images from 3 different SD cards to the max so far. Zero issues. Someone mentioned cfexpress.
Does anyone know if the slots read cfexpress A cards since they’re the same as sd cards/interchangeable in some cameras?
 
I'm working on my first photo edits since getting my fully loaded 16" M1 Max. After copying 300+ jpg files onto the hard drive, I opened them in Preview to have a quick look. To my surprise, and dismay, I got the spinning beach ball at least twice! Even my 2017 iMac never had an issue previewing photos this way. I do have an LG UltraFine 5K connected up but I don't believe this should cause any lag issues? Loading the raw files into Lightroom Classic was noticeably quicker however.
 
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