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In this article, we are going to identify the bottleneck of external storage drives and attempt to get the best possible data transfer speed out of your Akitio product.

To better understand what might be slowing down your drive and how you can improve the data transfer rate to get better speeds, we will look at each of the components separately.

Hard Disk Drives (HDD) and Solid State Drives (SSD)

The transfer speed can vary a lot depending on the drive model, even if it's the same type of drive, so it's always a good idea to check the specifications first and if possible, refer to an actual test result with that particular drive. In addition, there are certain limitations depending on the type of drive and its interface.

SATA HDD

A hard disk drive (HDD) uses one ore more rotating disks (platters) and due to the way the data is stored and accessed, such a drive is usually limited to a transfer rate of about 100-200 MB/s. You can stripe multiple drives to achieve faster speeds but in most cases, the bottleneck will be the drive itself.

SATA-III SSD

A solid state disk (SSD), at least with most models nowadays, can usually reach the limit of the SATA-III interface. The SATA-III interface, also known as SATA 3 or SATA 6B/s interface limits the transfer rate to about 550 MB/s.

TRIM

For a solid state disk (SSD), when using the drive without TRIM support over a longer period of time, the performance could be reduced. In order for TRIM to work, it has to be supported by all of the components, including the drive, the enclosure and the OS. If any of these components do not pass on the TRIM command, it will not work. It could be because the SSD doesn't support TRIM, because the OS doesn't support TRIM, because the SSD is part of a RAID or the TRIM command is not passed on to the drive due to the controller in the external enclosure.

PCIe SSD

A PCIe SSD can potentially reach much faster transfer rates than a SATA-III drive but the actual speed will depend on the model and type of drive (e.g. SATA, AHCI or NVMe based SSD). You can use a M.2 module or a PCIe drive in one of our Thunderbolt 2 or Thunderbolt 3 PCIe expansion chassis but there are a copule of things to keep in mind.

For the best performance, an NVMe based SSD will deliver the fastest speeds but at the time this article was written, macOS does not yet support 3rd party NVMe drives, so this works only on Windows (Update: macOS High Sierra and above now support 3rd party NVMe drives). Also, the speed of the drive could be limited by either the Thunderbolt or the PCIe interface. The fastest solution at this time is a Thunderbolt 3 PCIe expansion chassis with a Gen3 x4 PCIe interface and a Thunderbolt 3 connection, able to deliver a maximum of about 2750 MB/s. If the PCIe SSD is using a Gen2 x8 or Gen3 x8 interface, it will be limited to 4 lanes only.

Storage Interface

With the direct attached storage drives from Akitio, you often have multiples choices when it comes to the connection to the computer. For slower hard disk drives, the bottleneck is most likely the drive itself, so the interface doesn't make much of a difference but for faster SSDs and for multiple drives, the interface can make a big difference.

USB

Almost every computer has a USB port, making USB the ideal interface for drives that are used on more than just your own computer. For single HDDs, even the first generation of USB 3.1 (USB 3.0) is fast enough and will not limit your transfer rate. For SSDs, it's best to use the second generation of USB 3.1 at 10Gbps but for multiple drives, the transfer rate will be limited to around 700-800 MB/s and that's with the faster USB 3.1 Gen 2 interface.

Thunderbolt

With Thunderbolt 3, currently the latest generation of the Thunderbolt interface, you get plenty of bandwidth even for multiple drives and when daisy chaining additional Thunderbolt drives. The bottleneck of the Thunderbolt 3 interface is at around 2750 MB/s but for now, only certain NVMe SSDs can reach these kind of speeds, so in most cases, the transfer rate will not be limited.

With Thunderbolt 2, the bottleneck is at around 1375 MB/s. This kind of bandwidth is ideal for up to 4 SATA-III drives but it's not fast enough for an NVMe based SSD and even four SATA-III SSDs can be limited by this interface.

The first generation of Thunderbolt is similar to the USB 3.1 Gen 2 interface. The transfer rate will be limited to around 700-800 MB/s, which is ideal for multiple HDDs or 1-2 SSDs but not for more than 2 drives.

External Storage Enclosure

Aside from the interface, there are additional factors that could potentially slow down the transfer rate of your drives. To see what your enclosure is capable of, please refer to the benchmark results on the product page of your particular model but following are some things that can affect the disk speed.

Bus powered drives

Depending on the power requirements, under some circumstances, it might be necessary to throttle the read & write speeds of a drive when it's installed in a bus powered enclosure (e.g. Palm RAID or Neutrino Thunderbolt Edition). This is why a drive might run slightly slower than it usually would when installed inside a computer.

USB and RAID controller

When comparing the transfer rate of the same drive installed in the computer and in the external enclosure, there might be a slight difference in the transfer rate due to the electronics and overhead in the external enclosure. Even between different USB enclosures or between different RAID controllers, results may vary slightly.

PCIe to SATA controller

Not all Thunderbolt enclosures are built the same and to get the most speed out of a single SSD, you need to make sure that the SATA controller provides enough bandwidth to let the drive run at full SATA 6B/s speeds. For more information, please refer to this article about making full use of the SATA 6Gb/s interface.

The PCIe to SATA controller does not only affect the speed of a single drive but also limits the transfer rate of certain enclosures like the Thunder3 Quad and the Thunder3 Quad Mini. These 4-bay enclosures use either 4x PCIe to SATA controllers with one Gen-2 PCIe lane per controller for the Thunder3 Quad or 2x PCIe to SATA controllers with two Gen-2 PCIe lanes per controller for the Thunder3 Quad Mini, limiting the total transfer rate of the internal disks to about 1480 MB/s, even though the Thunderbolt 3 interface would be capable of faster speeds. You can still daisy chain additional Thunderbolt 3 devices and make use of the full 40Gbps bandwidth though.

Cables and adapters

For slower interfaces like USB 3.1 Gen 1, the cable usually doesn't affect the transfer rate, so any cable should work fine. For faster interfaces like USB 3.1 Gen 2 and Thunderbolt, the type and quality of the cable can affect not only the transfer rates but also the functionality, so it's important to always use the cables that came included with your product.

When buying a different cable, as a general rule, the shorter and thicker the less problems and the longer the cable gets, the more likely the quality can affect the speed or functionality. Special care has to be taken when buying an adapter plug or a cable to convert one type of connector to another (e.g. USB-C to USB Type A). A bad adapter can cause the drive to lose connection and some adapters are not designed to handle the faster transfer rates of interfaces like USB 3.1 Gen 2 or Thunderbolt 3 for example.

Computer

There are simply too many different combinations and components to predict how exactly your computer will affect the transfer rate of the external storage device. Suffice to say that when testing the drive on different computers, results will vary. However, following are some poinst to consider.

Connectivity

Always connect the external drive to an on-board port. Front extensions and other ports not directly on the motherboard can affect the transfer rate.Connecting the device behind a hub or in a daisy chain can also affect the speed.

Source & Destination

If you are using a fast external drive (e.g. SSD) but the source or destination used for the file transfer is a slower drive (e.g. HDD), the faster drive won't be able to reach its potential.

Shared resources

Depending on the hardware design of the motherboard, resources might be shared with other components and if all of it is in use at the same time, it can affect the transfer rates of your external drive. For example, a computer with two Thunderbolt ports is likely using only one Thunderbolt controller and if you are using another Thunderbolt device on the second port at the same time, the bandwidth will be shared. The exception to this case is the Mac Pro (2013), which has multiple Thunderbolt controllers. Another example are the PCIe slots and the M.2 slots. Make sure to read the documentation of your motherboard, so that you can avoid reduced bandwidths for something like a Thunderbolt PCIe expansion card when a M.2 module is installed at the same time. In some rare cases, even something like enabling WiFi and Bluetooth on the latopop can affect the Thunderbolt interface.

Limited PCIe lanes

Some Thunderbolt 3 laptops are designed with only two PCIe lanes for the Thunderbolt interface. To reach the maximum of the Thunderbolt 3 interface at 40Gbps, four PCIe lanes are required, so having only two lanes will effectively cut the speed in half and you will only get 20Gbps. For laptops from Dell, check this page at www.dell.com. For laptops from Lenovo, have a look at this article on www.reddit.com. For other laptops, check your documentation, do a search on Google or contact the manufacturer for more information.

Benchmark Results

When testing the transfer rate of your external drive, there are several ways to do it. You could transfer some files and measure how long it takes to complete or you could use a benchmark tool to test the disk speed. Either way, the testing procedure can affect the results you are getting and it might be slower or faster than what you see on our website. If possible, try different settings and different tools to get a better idea of what your drive is capable of.

Test Environment

Last but not least, there are several other factors that can affect the transfer rate of your external drive. Following are some examples of what to look out for.