Why USB and SD Card have different Speed?

Many of us always ask Why there is difference in writing and reading speed of an SD Card and USB Drive…

I think the question mixes up two separate memory device technologies, i.e. SD cards and USB drives both which operate on Flash memory. 

1. USB drives operate on speeds mandated by USB protocol. USB 2.0 corresponds to 480 Mbps (Megabits per sec.) (60MBps) and USB 3.0 corresponds to 5 Gbps.USB sticks are faster than SD cards as the USB protocol, USB bus frequency, are modified to support higher speeds.

2. SD cards are determined by their capacities and speeds. The speed class eg Class 2. Class 4, mentions the speed of sequential read / write operation.  A class 2 SD card offers minimum 2MBps (Megabyte per sec.) sequential read / write performance, similarly, Class 4 offers 4MBps (Megabyte per ssec.), and Class 10 offers 10MBps (Megabyte/s) guaranteed.  In fact, there is Class 0 also, which does not guarantee any performance.

In addition to this, the speed class of a card is also linked to card capacity. There are three kinds Capacity of Memory

(1) Standard Capacity SD Memory Card (SDSC): Up to and including 2 GB
(2) High Capacity SD Memory Card (SDHC): More than 2GB and up to and including 32GB
(3) Extended Capacity SD Memory Card (SDXC): More than 32GB and up to and including 2TB

Both SDHC and SDXC can offer speeds anywhere from Class 2 to Class 10. SDSC at just Class 2. Nowadays there are newer protocols called UHS-1 / UHS-2 (Ultra High Speed), which operate with lower power requirements and offer speeds close to USB.

 

The U symbol in image below indicates UHS card, the (10) indicates Class 10 and you can also see SDHC.  The SanDisk card offers 45 MBps read /write which is much more than Class 10 requirement.

I hope this may help you.