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Why can't I reach the advertised performance of my external drive?

 

Many factors can affect the performance of an external drive including the connection, the cable, and the type of device. 

An important fact to remember is that the data transfer will go as fast as the slowest device. This means if data is being transferred from a slower source (HDD) to a faster destination (SSD) the maximum achievable transfer rate is limited by the source drive.  

Another common factor of slow performance relates to the file size or file type. Data transfer rates of small files are usually slower than speeds achieved while transferring large files.  

This article provides information on some factors that may impact the performance of an external drive. 

 

Enabling write caching in Windows 

If you are using your external drive in Windows, the performance can be improved by enabling write caching. For instruction visit How to improve performance of an external drive in Windows

 

Speed drops during a transfer 

When transferring data to your external drive you may experience higher transfer rates, also known as burst rate, at the beginning and then at a certain point you may notice the transfer speed slowing down. This happens because the data is written to the destination drive’s cache memory first, so once the cache is full the performance slows down. Also, as mentioned before, the file type or file size may impact data transfer speeds. For example, if during a transfer you experience a decrease in speed, it could be that small files are being transferred and that larger ones were being transferred before. 

 

Interface 

There are many interfaces available so it’s important to know if your computer supports the interface of your external drive. Check the computer documentation or contact the manufacturer for more information.  
Below is the maximum throughput of the most common interfaces:  
 

Interface 

(Gigabits per second) Transfer Rates (MegaBytes per second) 

Thunderbolt 3 

up to 40Gb/s 

or 

up to 5,000 MB/s 

Thunderbolt 2 

up to 20Gb/s 

or 

up to 2,500 MB/s 

Thunderbolt 

up to 10Gb/s 

or 

up to 1,250 MB/s 

USB 3.2 Gen 2x2 

up to 20Gb/s* 

or 

up to 2,500 MB/s 

USB 3.1 Gen 2 

up to 10Gb/s 

or 

up to 1,250 MB/s 

USB 3.1 Gen 1 

up to 5Gb/s 

or 

up to 625 MB/s 

SuperSpeed USB 3.0 

up to 5Gb/s 

or 

up to 625 MB/s 

Hi-Speed USB 2.0 

up to 480Mb/s 

or 

up to 60 MB/s 

Note: USB3.2 Gen 2x2: two lanes at 10Gb/s. 

 

Connectivity 

To achieve the best performance, it’s recommended to directly connect the external drive to your computer. Adapters, hubs and other types of expansion solutions can affect the performance of an external drive. 
If the external drive has multiple interfaces, make sure to connect the right cable to the right port. Here is an example: if an external drive has a Thunderbolt 3 port and a USB-C 3.1 port but you are connecting a USB-C 3.1 Gen 2 cable to the device's Thunderbolt 3, the maximum throughput will be 10Gb/s. So, it’s important to check if the cable connected to the device is appropriate. 
  

Cable 

It’s recommended to use the original cable that came with your external drive as many third-party cables may use different protocols or can be poorly made which can affect performance or even damage your drive or your computer. 

Note: There are two types of Thunderbolt 3 cables: Active and Passive 
  • Passive cables are usually longer, less expensive and can reach up to 20 Gb/s
  • Active cables are faster, can reach up to 40Gb/s, are more expensive and are available in different sizes. 
  

Resources

Your computer plays a significant role in the performance of an external drive. Depending on the hardware of the computer, resources might be shared with other components and if all the resources are in use at the same time, it can affect the transfer speeds of your external drive.  
Some Thunderbolt 3 computers, for example, are designed with two PCI lanes but in order to reach 40Gb/s allowed by Thunderbolt 3, it requires four PCI lanes. Meaning if the computer has only two lanes the maximum it can reach is 20Gb/s. The MacBook Pro 13-inch late 2016 is a great example of this limitation. For additional details visit Slow performance with the late-2016 MacBook Pro. Also, some Dell Thunderbolt 3 systems are designed differently.  Some models are made with two PCI lanes and some with four PCI lanes, see here additional details. 
When doing large data transfers or trying to maximize the overall speed of the transfer, it is best to ensure that any other tasks are held to a minimum. This will allow your system to put more focus on the data transfer which should increase its performance. 

 

File System 

You can optimize performance by formatting your storage device using the computer’s native file system. If you intend to use your storage device only on Mac, it’s best to format Mac OS Extended (Journaled) also known as HFS+ or APFS. For Windows, it would be best to format NTFS.  If you need to use your device on both Mac and PC then it’s best to format exFAT,  but you may not get the best transfer rate as it's not the optimized file system for either operating system. For additional information on how to format your storage device visit How to format your hard drive

 

Source and Destination 

If the data transfer comes from a source drive that is slower than your external drive, the transfer rate is affected since it is limited by the slower drive. Also, if the storage space is nearly full, this can result in slower performance too.  

 

HDDs and SSDs 

Several types of external drives are available today. Some are made using the traditional spinning hard disk drive (HDD) and some are made using the faster technology solid state drive (SSD). The devices are made with different types of interfaces so among other factors the performance of an external drive is determined by the technology used to store data and the type of interface the device has.  
See below for the main characteristics of each technology: 
  
HDD
Composed of moving parts with one or more platters, heads, and other components, HDDs magnetically store data on the platters (disks). HDDs are made in two form factors, 3.5” and 2.5” with different spin speeds or RPM (rotation per minute) varying from 5400 RPM to 7200 RPM. Most recent HDDs use a SATA connection. 
 
SSD
Unlike HDDs, an SSD doesn't have moving parts, it’s composed of NAND flash memory providing faster performance, noiseless operation and reliability. There are different types of flash memory meaning the performance of an SSD will be based on the type of flash being used. Also, the performance can change over time as it will depend on read/write history and the type of stimulus applied to the drive (IO requests). Usually, the newer the drive the better its performance. See below the main differences between types of flash memory: 
 

NAND flash types 

 

Pros 

Cons 

SLC 

Single Level Cell 

single bit of data per cell 

 

Enterprise grade solution 

Faster performance 

Most accurate data read and write. 

Low density (1 bit per cell) 

Low power consumption 

Lifespan High - cycles ~90,000 - 100,000 

Most expensive  
 
 

eMLC 

Enterprise Multi Level Cell 

multi bits of data per cell 

 

Enterprise grade solution 

Performance - faster than MLC 

Cost less than SLC 

Last longer then MLC - cycles ~20,000 - 30,000 

Optimized for enterprise 

Performance - slower than SLC 

High density (2bits per cell) 

MLC 

Multi-Level Cell 

multi bits of data per cell

 

Consumer /Gaming grade solution 

Less expensive than SLC 

More reliable than TLC flash 

 
 
 

Performance - slower than SLC  

Less accurate data read and write  

High density (2bits per cell) 

Higher power consumption 

Lifespan Low - cycles ~10,000 

TLC 

Triple Level Cell  

three bits of data per cell 

 

Consumer grade solution 

Low cost 

Performance - slower than MLC  

High density (3bits per cell)  

Lifespan Low - cycles ~3,000 -5,000  

 

QLC 

Quad Level Cell 

four bits of data per cell 

 

Consumer grade solution 

Low cost 

 

Performance – slower than any other 

High density (4bits per cell) 

Lifespan Low – cycles ~1000 

 
Most recent versions of HDDs use a SATA connection, but for SSDs different technologies are available, see below:  

SATA III – also known as SATA 6Gb/s is the third generation SATA interface runs at 6Gb/s and throughput of 600MB/s. 

PCIe (peripheral component interconnect express) - this interface is commonly used to connect components directly to a computer's motherboard such as video cards, RAID cards, etc. But recently SSDs became available using this interface. PCIe has many versions but currently, SSDs are being manufactured using PCIe gen 3 supporting a bandwidth of 32GB/s and gen 4 supporting 65GB/s. 

M.2 - also known as NGFF (Next Generation Form Factor) offers versatility and flexibility as it supports SATA III and PCIe connections and is made in different sizes. The most common is M.2 2280 measuring 80 x 22 mm. 

NVMe - NVMe (Non-Volatile Memory Express) is a protocol specifically designed for SSDs that allows communication between the controller and the storage components, optimizing performance. This technology is available in different form factors: U.2 which exclusively uses NVMe, PCIe, and M.2. NVMe was designed to explore the potential beyond what was achieved by AHCI (Advanced Host Controller Interface) used with SATA. NVMe increases the ability to receive read and write commands simultaneously, promoting lower latency, energy-saving, and most importantly, improving performance. 

 

RAID 

Some external drives may have RAID capabilities which may also affect performance. For example, with RAID 0, you should typically expect better speeds than a single drive, but with RAID-1 you aren't likely to see many real performance improvements. 
 
Examples of RAID Configurations 
 

Standard RAID  

Mode 
Min. # disks 
Data Protection 
Fault Tolerance 
Performance 
Read        Write 
Capacity Utilization 

RAID 0 

2 

No 

0 disk 

High 

High 

100% 

RAID 1 

2 

Yes 

1 disk 

High 

Medium 

50% 

RAID 5 

3 

Yes 

1 disk 

High 

Low 

67% - 94% 

RAID 6 

4 

Yes 

2 disks 

High 

Low 

50% - 88% 

Nested RAID 

RAID 10 

4 

Yes 

1 disk per nest 

High 

Medium 

50%