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When I read the information about an SD card, I notice there is an information that is reported as 100x. What does that mean? To what is it referring?

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up vote 12 down vote accepted

The x rating on an SD card is a measurement of read/write speed. 1x means a speed of 1.2 Mb/s, so 100x would mean a maximum speed of 120 Mb/s (or 15 MB/s). Whether or not this refers to maximum write speed or maximum read speed is vendor-specific.

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I guess they will report the read access speed, to make think the SD card is more performant than it is. :-) Thank you for your reply. – kiamlaluno Aug 23 '10 at 23:52
    
It should also be noted that the speed is pretty much always given as a maximum (as in up to) as opposed to average speed. – Synetech Jan 23 '12 at 2:57
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To be precise, the convention is that the speed is given in the same format as for CD drives, so 1x is as gP says 1.2 Mb/s or the perhaps more known number 150 kB/s. – nitro2k01 Aug 14 '12 at 0:55

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