01Feb USB 3.0 card reader speeds and why it matters

This question came in and I thought it might be helpful if I answer it on the site.

 Since USB 3.0 has a max transfer speed of 5 Gbps and USB 2.0 has a max of 480 Mbps, and SD class 10 cards have a minimum speed of 10 Mbps, does it really make any practical sense to upgrade to a USB 3.0 card reader since the card itself will always be the bottleneck?

Some of the confusion stems from “bits” vs “bytes”, but there is also a little more to it. First of all a bit is a single numeric value, either a 1 or a 0, while a byte is made up of 8 bits. These are usually notated with a lowercase “b” for bits and an upper case “B” for bytes.

In this case USB 2.0 provides a maximum of 480 megabits per second transfer speed. That gives you a max transfer speed of 60 megabytes per second. However USB 2.0 is half-duplex, which means it can only send data one direction at a time, in other words you can’t write to and read from a card simultaneously. This drops the actual data transfer speeds to and from the card down to between 25-40 megabytes per second. On top of that these speeds are further limited by the fact that USB 2.0 specifications apply to the USB controller not the actual port and there are usually many USB ports tied to the same controller. If you have more than one device plugged in, transfer speeds are spread across multiple devices. With a USB keyboard and a few other random things attached you’ll probably see speeds closer to 20 megabytes per second or lower.

So lets take a look at what this means. In this case i’ve tested this middle of the line (in terms of speed) Sandisk Extreme 45MB/s card. It’s not the fastest card on the market, but it’s not slow by any means and does a good job of keeping up with DSLR video recording. I’ve setup the test to write 3GB to the card then read 3GB from the card to verify the data. 

As you can see from this H2testw write test, the card is averaging 17.7 megabytes per second connected to a USB 2.0 card reader on my editing bay and reading from the card at 18.7 megabytes per second. That’s not horrible in terms of write speeds but it’s far less than the 45MB/s advertised. Now lets test that exact same card on the Transcend USB 3.0 card reader.

The Sandisk Extreme connected to the Transcend USB 3.0 card reader running the exact same test is now delivering 33.7 megabytes per second write speeds and read speeds of 38 megabytes per second. With the USB 2.0 card reader 2 min. 40 sec. to read 3GB worth of data on the Sandisk Extreme card, while the USB 3.0 card reader will give you 3GB in 1 min. 18 sec. which is roughly double the USB 2.0 card reader speed.

Double the speed when ingesting footage is a significant difference, and it becomes even more extreme when using cards with read and write speed ratings in the 90 MB/s and above range. With this Sandisk Extreme 45MB/s card alone you’ll be able to to copy a full card in 13 minutes instead of 26 minutes. That’s a pretty decent increase for a $14 investment in a USB 3.0 card reader.

Tags: speed test, Transcend card reader, USB 3.0 card reader