Quick Answers
The write speed of an external hard drive can vary quite a bit depending on the drive’s interface type, form factor, and specific model. Some key factors that affect write speed include:
- Interface – Faster interfaces like Thunderbolt 3 and USB 3.2 Gen 2×2 can offer write speeds up to 2,800 MB/s, while older USB 2.0 maxes out below 60 MB/s.
- HDD vs SSD – Solid State Drives (SSDs) offer much faster write speeds, generally over 500 MB/s, compared to Hard Disk Drives (HDDs) which max out below 200 MB/s.
- Form factor – Smaller form factors like 2.5″ drives tend to be slower than larger 3.5″ drives. But some smaller SSDs can still be very fast.
- Cache size – Drives with a larger cache size can optimize writes and maintain faster write speeds.
Overall, modern external SSDs over USB 3.2 Gen 2×2 or Thunderbolt 3 can reach peak write speeds over 2,500 MB/s. But economical external hard drives are often in the 100-200 MB/s write speed range.
External hard drives provide a convenient way to add storage space to your computer or easily move large amounts of data between devices. With external storage being so commonplace today, write speed is an important specification to consider when selecting a drive.
The write speed determines how quickly data can be saved or transferred to the external drive. Faster write speeds allow you to move files onto the drive more rapidly or reduce wait times when saving large files like videos. Slower write speeds can lead to bottlenecks and extra time staring at the progress bar.
In this article, we’ll take a deep dive into the factors that affect real-world write performance for external hard drives. We’ll also look at write speeds for common drive types, interfaces, and form factors. Let’s start by looking at some of the key specifications that determine write speed.
Key Factors That Affect Write Speed
There are several important factors that play a role in determining the real-world write speed of an external drive, including:
- Interface – The interface between the external drive and computer, such as USB, Thunderbolt, etc. This sets the maximum theoretical bandwidth.
- Drive type – Whether the drive uses an HDD, SSD, or hybrid design. HDDs and SSDs have vastly different write capabilities.
- Form factor – The physical size and shape of the drive. Bigger drives allow for faster write speeds.
- Cache size – The amount of fast cache memory on the drive controller. More cache can help optimize writes.
- Controller – The drive controller and firmware optimizations also impact achieved write speed.
We’ll take a closer look at how each of these factors influence the real-world write performance you can expect from an external drive.
Interface Type Sets the Maximum Speed
The interface used to connect the external drive to your computer plays a major role in determining write performance. Each interface type has a maximum theoretical bandwidth that sets the upper limit for transfer speeds.
Common external drive interface types include:
- USB 2.0 – Up to 60 MB/s write speed
- USB 3.0/3.1 Gen 1 – Up to 625 MB/s write speed
- USB 3.1 Gen 2 – Up to 1,000 MB/s write speed
- USB 3.2 Gen 2×2 – Up to 2,000 MB/s write speed
- USB 4 – Up to 4,000 MB/s write speed
- Thunderbolt 3 – Up to 2,800 MB/s write speed
- Thunderbolt 4 – Up to 3,000 MB/s write speed
As you can see, the interface alone can mean the difference between a maximum write speed of 60 MB/s over USB 2.0 versus up to 4,000 MB/s with USB 4 or Thunderbolt 4. Keep in mind that real-world write speeds are always lower than maximum theoretical interface bandwidth.
HDD vs SSD – A Game Changer for Write Speeds
The type of storage technology used by an external drive has a huge impact on achievable write speeds. Hard disk drives (HDDs) and solid state drives (SSDs) offer vastly different write performance profiles:
- Hard Disk Drives (HDDs) – HDDs write data to spinning platters with magnetic read/write heads. The mechanical nature limits most external HDD write speeds to between 100-200 MB/s at best.
- Solid State Drives (SSDs) – SSDs have no moving parts and write data to flash memory chips. External SSDs can achieve write speeds well over 500 MB/s, with peak speeds over 2,500 MB/s over the fastest interfaces.
In most cases, you can expect around a 5-10x increase in real-world write speeds by going from an HDD to SSD. However, SSDs are more expensive per gigabyte compared to HDDs.
Physical Size and Form Factor
The physical size and form factor of an external drive can also have an impact on write performance. Bigger drives allow for more platters and read/write heads (for HDDs) or more flash memory chips (for SSDs). This can translate to faster write speeds compared to smaller drives.
Some common external hard drive form factors include:
- 3.5-inch – The largest consumer hard drive size, typically used for desktop external drives. 3.5″ HDDs offer superior write speeds compared to smaller drives.
- 2.5-inch – A smaller form factor used for portable external drives. 2.5″ drives tend to have lower write speeds, but SSDs in this form factor can still be very quick.
- M.2 – A form factor commonly used for internal SSDs, M.2 drives are now also available in ultra-compact external drive enclosures.
While the 3.5″ form factor offers faster write speeds for HDDs, some high performance SSDs in smaller 2.5″ or M.2 formats can outpace larger HDDs. An SSD in a smaller form factor usually provides the best balance of speed and portability.
Cache Size Matters for Speed Optimization
The cache or buffer on the drive’s controller board also plays an important role in optimizing real-world write performance. Data is first written to the fast cache, often DRAM or NAND flash memory, before being written to the actual storage medium.
Larger cache sizes help maintain faster write speeds by queueing up more data in fast cache memory before it has to be written to disk. A large cache is particularly beneficial for HDDs which have slower mechanical write speeds.
For example, an HDD with 64MB of cache will typically be able to write data faster and more consistently than a drive with only 8MB of cache. For SSDs, larger cache sizes also aid sustained write performance.
Write Speeds by Interface and Drive Type
Now that we’ve looked at the key factors that affect real-world write speeds, let’s see some examples of speeds you can expect with different interface types and storage mediums. Here are typical write speeds based on empirical test data and manufacturer specifications.
External HDD Write Speeds
| Interface | Typical Write Speeds |
| USB 2.0 | 25-40 MB/s |
| USB 3.0/3.1 Gen 1 | 80-120 MB/s |
| USB 3.1 Gen 2 | 100-200 MB/s |
| Thunderbolt 1/2 | 150-200 MB/s |
As you can see, USB 2.0 severely bottlenecks HDD write speeds. You’ll want at least USB 3.0 for decent performance, with Thunderbolt or the latest USB interfaces allowing external HDD writes near the drives’ native limits.
External SSD Write Speeds
| Interface | Typical Write Speeds |
| USB 3.1 Gen 1 | 300-500 MB/s |
| USB 3.2 Gen 2 | 600-1,000 MB/s |
| USB 3.2 Gen 2×2 | 1,000-2,000 MB/s |
| Thunderbolt 3 | 1,500-2,500 MB/s |
Even over older USB 3.0/3.1 Gen 1 connections, external SSDs offer impressive write speeds. The latest interfaces like USB 3.2 Gen 2×2 and Thunderbolt 3 allow bleeding-edge SSDs to nearly saturate the interface bandwidth when writing data.
Max Write Speed Depends on File Size
Another important point to note is that sustained write speed is heavily dependent on file size. When writing large multi-gigabyte files, drives can achieve much closer to their interface or disk limit speeds.
But when writing smaller files a few megabytes or less in size, the write speed will be bound by other limiting factors like seek time and latency rather than pure interface bandwidth.
For example, a USB 3.2 Gen 2 SSD may be able to write a single large 5GB file at 900 MB/s. But its speed for writing hundreds of 4KB files may drop down closer to 100 MB/s.
So keep in mind that maximum interface write speeds are typically only reached when transferring very large files. For smaller everyday file operations, average speeds will usually be lower.
Seek Time and Access Latency
In addition to sequential write throughput, seek time and access latency are other important performance factors that relate to write speeds:
- Seek time – The time required for the read/write head on an HDD to locate data on the drive platters. Seek time impacts write speeds for lots of small file writes.
- Access latency – The delay for requested data to be retrieved from the drive. SSDs have very low latency which benefits small file writes.
SSDs have a huge advantage over HDDs when it comes to seek time and access latency. While HDDs must physically move a read/write head to locate data, SSDs can access data electronically with minimal delay. This gives SSDs a major speed boost for small file random writes.
RAID Array Write Speeds
With enterprise and higher-end consumer storage, multiple drives can be configured together in a RAID array for increased performance or redundancy.
Common RAID levels like RAID 0 and RAID 10 boost write speeds by striping and parallelizing writes across multiple drives. For example:
- RAID 0 – Doubles write speed by striping data in parallel across two identical drives with no redundancy.
- RAID 10 – Doubles read speed and maintains redundancy by mirroring two striped drive pairs.
RAID arrays containing multiple SSDs or HDDs can achieve very high collective write speeds not possible with a single drive. But RAID involves more complexity and only offers speed benefits in certain workloads.
Conclusions
To summarize, interface type, storage medium, form factor, and cache size all play major roles in determining the real-world write speeds of an external hard drive. Modern external SSDs over fast interfaces like USB 3.2 Gen 2×2 or Thunderbolt 3 offer incredible write speeds up to 2.5-2.8 GB/s.
On the more affordable end, typical external HDDs are limited to 100-200 MB/s writes over USB 3.0/3.1 Gen 1. Going with an external SSD provides a massive boost in write performance. But HDDs offer more affordable bulk storage.
When shopping for an external drive, be sure to consider the interface and whether solid state or mechanical hard drive technology will work best for your performance needs and budget.