Before we get deep into testing how fast or slow an external drive or USB flash drive is, let us first deal with the basic facts. Internal drives by design are much faster than external drives. That said if you plan to use an external disk as a replica for your internal disk. You should not expect the same performance.
Nonetheless, it is important to establish the write and read speed of an external drive so that you can weigh if it will be able to run the app or game you would like to install. The trick shared below is for Mac OS X users (this time round, sorry to our Windows users).
We are going to establish the read and write speed for USB flash thumb drive, and the standard USB external hard diskS, A Thunderbolt-based external disk, firewire, and even network volumes.
PS: If you want to decide between two or more external storage for backing up your Mac using Time Machine, or for RAID setups. This procedure will go a long way in helping you determine which one has the fastest read and write speeds.
The drive you are testing needs to be formatted for use with Mac OS X. If it is formatted for use with Windows or both Mac and Windows, the speed test will reveal a lower value than if it were formatted
Disk Speed Test app
The first app we are going to use to test on write and read speed of an external disk or USB thumb drive is the Disk Speed Test app (available as free download from Mac App Store).
After installing the app on your Mac, plug in the external drive (USB, Firewire, Thunderbolt, etc). Launch the app and click the gear button and click on Select Target Drive.
Test using Xbench
Another popular app for determining the read and write speed of external storage is the Xbench. Also available as free download from this link. Xbench comes with a much simpler interface compared to the one mentioned above; then again, you only need it to test the speed of the drive.
After installing Xbench on your Mac, launch it and uncheck all the option except for Disk Test. Next, you will need to pull down the Volume menu and select the external drive from the resulting list. Next, click on Start to begin the test.
Xbench will access the full disk read and write speed, and go ahead to display a benchmarking test. Once the process is complete, you will see the results displayed under the ‘Disk Tests’ in sequential order of write and read. There will also be random results on read and write speed for the different size file blocks.
What determines the speed of a Drive
There are many factors that come into play determining the read and write speeds of a drive. First, there is the connection interface; USB, USB 2, Thunderbolt etc. The type of disk also matters, with SSD being much faster than traditional rotating hard drives.
Like mentioned earlier, hard disks designed from the go to be internal storage work much faster than those designed to be external storage. There is also the other matter of generation, with new SSDs and hard disks performing much faster than their counterpart from an older generation.