I'm using a Pi Zero W with a micro SD HC and a Pi4 with a XC-I card. My questions: whats the fastest micro SD card the Pi Zero and Pi 4 will support? I'm aware that the Pi Zero can read XC but will it be at full speed? Would the Pi4 / zero support full speed for a XC-II card?

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    If you look around at benchmarks online, it seems to max out at 40-50 MB/s, and higher rated cards are not necessarily better. With a Sandisk Ultra (UHS-I/class 10, rated at 80 MB/s) I get ~50 MB/s for an idealized write (dd if=/dev/zero of=test bs=4M count=256). – goldilocks Dec 22 '20 at 15:26
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    The speed ratings of SD Cards are related to burst writing i.e. Video. This has nothing to do with the way the Pi works (random R/W), but anything beyond SD10 will make no difference – Milliways Dec 22 '20 at 23:03
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    raspberrypi.org/blog/sd-card-speed-test may be of interest – Milliways Dec 24 '20 at 0:18

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