View source for Understanding SD Card Ratings

The package for a SD card may say Ultra High Speed Pro, but the markings on the card tell the truth. These markings are defined by the SD Card association. The write speed is slower than the read speed, since the memory must be cleared before writing new data.

== Capacity markings ==

SD up to 2 Gbytes

SDHC (High capacity) 4G to 32G

SDXC (Extended capacity) 64G to 2Terabytes

== Standard SD Cards ==
The maximum standard SD interface speed is 25 MBytes/second.

'''Write speed markings'''

circle 2 - 2 Mbytes/sec

circle 4 - 4 Mbytes/sec

circle 6 - 6 Mbytes/sec

circle 8 - 8 Mbytes/sec

circle 10 - 10 Mbytes/sec

'''Interface speeds'''

(1) Default Speed mode: 3.3V signaling, Frequency up to 25 MHz, up to 12.5 MB/sec

(2) High Speed mode: 3.3V signaling, Frequency up to 50 MHz, up to 25 MB/sec

== UHS SD Cards ==
The UHS-I and UHS_II specifications adds faster interface speeds. Note that the host controller must support these interface options for the faster interfaces. These higher speed cards are backwards compatible, so you can use them with older hardware. The UHS-I interface runs at up to 104 Mbytes/sec, and the UHS-II up to 312Mbyte/sec.

'''Write speed markings'''

U1 - 10 Mbytes/sec write speed

U3 - 30 Mbytes/sec write speed

'''Interface speeds supported'''

(1) Default Speed mode: 3.3V signaling, Frequency up to 25 MHz, up to 12.5 MB/sec

(2) High Speed mode: 3.3V signaling, Frequency up to 50 MHz, up to 25 MB/sec

(3) SDR12: UHS-I 1.8V signaling, Frequency up to 25 MHz, up to 12.5MB/sec

(4) SDR25: UHS-I 1.8V signaling, Frequency up to 50 MHz, up to 25MB/sec

(5) SDR50: UHS-I 1.8V signaling, Frequency up to 100 MHz, up to 50MB/sec

(6) SDR104: UHS-I 1.8V signaling, Frequency up to 208 MHz, up to 104MB/sec

(7) DDR50: UHS-I 1.8V signaling, Frequency up to 50 MHz, sampled on both clock edges, up to 50MB/sec

(8) UHS156: UHS-II RCLK Frequency Range 26MHz - 52MHz, up to 1.56Gbps per lane.

(9) UHS312: UHS-II RCLK Frequency Range 26MHz - 52MHz, up to 1.56Gbps two lanes half duplex

The UHS class 3 devices are available from a few vendors for about the same price, why would you buy anything else?

Link to the SD cards tested with a Raspberry PI. [http://elinux.org/RPi_SD_cards#SD_card_performance]

Last edited 5-25-2015