Hard drives have varying speeds depending on the interface they use to connect to a computer and their internal components. The speed of a hard drive is commonly measured in megabytes per second (MB/s) or megabits per second (Mbps).
Measuring Hard Drive Speed
There are a few key factors that determine the speed of a hard drive:
- Interface – The interface between the hard drive and computer determines the maximum theoretical bandwidth. Common interfaces include SATA, SAS, USB, and Thunderbolt.
- Spindle speed – The spinning platters inside a traditional HDD have a rated speed in revolutions per minute (RPM). Faster RPMs allow data to be accessed more quickly.
- Cache size – The cache acts as fast temporary storage, so a larger cache can improve read/write speeds.
- Average seek time – The average time it takes the read/write heads to move and find data on the platters.
SATA Interface Speed
The Serial ATA (SATA) interface is the most common in desktop computers. SATA has steadily increased in speed over the years:
|SATA Version||Speed (Gbit/s)||MB/s||Mbps|
As you can see, the speed has increased from 150 MB/s to 600 MB/s from SATA 1.0 to 3.0. Actual real-world speeds are a bit lower due to protocol overhead and depends on the specific drive, but this shows the theoretical maximum bandwidth.
7,200 RPM vs 5,400 RPM HDDs
The spindle speed of a traditional hard disk drive (HDD) greatly impacts its speed. 7,200 RPM drives are generally faster than 5,400 RPM drives. Here’s an example compare of two SATA 3.0 HDDs from Seagate:
|Model||Spindle Speed||Interface||Cache||Max Sustained Transfer Rate|
|BarraCuda Pro 7200RPM||7,200 RPM||SATA 6 Gb/s||256 MB||249 MB/s|
|BarraCuda 5400RPM||5,400 RPM||SATA 6 Gb/s||256 MB||140 MB/s|
With all else being similar, the 7,200 RPM drive provides significantly faster sustained transfer speeds of 249 MB/s vs 140 MB/s on the 5,400 RPM drive. This shows the impact of spindle speed on HDD performance.
SSD vs HDD Speed
Solid State Drives (SSDs) use flash memory and have no moving parts. This allows them to achieve much faster speeds than traditional HDDs. A SATA SSD can have sustained reads/writes over 500 MB/s, compared to HDDs which top out between 100-250 MB/s.
Here’s an example speed comparison between a SATA SSD and HDD from Samsung:
|Model||Type||Interface||Max Sequential Read||Max Sequential Write|
|860 EVO SSD||SSD||SATA 6 Gb/s||550 MB/s||520 MB/s|
|Spinpoint M8 HDD||HDD||SATA 6 Gb/s||210 MB/s||187 MB/s|
The SSD provides over double the sequential read/write speeds. So while a high performance HDD may reach ~250 MB/s, even budget SATA SSDs will exceed 500 MB/s.
NVMe SSD Speed
NVMe (Non-Volatile Memory express) is a newer SSD interface that was designed from the ground up for SSDs, unlike SATA which was originally designed for HDDs. NVMe drives connect via PCIe directly to the system bus, allowing for much higher bandwidth.
Here is an example NVMe SSD compared to a SATA SSD:
|Model||Interface||Max Sequential Read||Max Sequential Write|
|Samsung 970 EVO Plus NVMe SSD||NVMe PCIe 3.0 x4||3,500 MB/s||3,300 MB/s|
|Samsung 860 EVO SATA SSD||SATA 6 Gb/s||550 MB/s||520 MB/s|
With PCIe 3.0 x4 bandwidth, the NVMe drive provides over 6x higher sequential read/write speeds compared to the SATA SSD. NVMe drives truly unlock SSD performance by removing the SATA bottleneck.
RAID (Redundant Array of Independent Disks) allows combining multiple drives together for increased performance or redundancy. Common RAID levels include:
- RAID 0 – Stripes data across disks for faster reads/writes but no redundancy.
- RAID 1 – Mirrors disks for 100% redundancy but no speed boost.
- RAID 5 – Stripes data and adds parity for redundancy and some speedup.
- RAID 10 – Mirrors and stripes for speed and redundancy.
For example, two 250 MB/s HDDs in RAID 0 can deliver nearly double the speed at 500 MB/s. But if one drive fails, all data will be lost. RAID 5 provides redundancy while still allowing some speedup. RAID 10 combines mirroring and stripping for both speed and redundancy.
External Hard Drives
External portable hard drives usually connect via USB and spin at 5,400 RPM. Here are some typical speeds:
|Model||Interface||Spindle Speed||Max Sustained Transfer Rate|
|WD Elements Portable||USB 3.0||5,400 RPM||100 MB/s|
|Seagate Backup Plus Slim||USB 3.0||5,400 RPM||120 MB/s|
The USB 3.0 interface caps at 640 Mbps or 80 MB/s theoretical bandwidth. Actual real world speeds end up around 100-120 MB/s for these drives. USB 2.0 is much slower at 60 MB/s max.
There are some portable SSDs available which connect via USB 3.0 or USB-C and can reach over 400 MB/s speeds despite the USB bottleneck.
Hard Drive Access Time
In addition to sustained transfer rates, another important metric for hard drive speeds is access time. Access time measures how fast a drive can locate and retrieve a piece of data.
Access time is broken down into two main components:
- Seek time – How long it takes the read/write heads to move into position over the correct track on the platter.
- Latency – The delay waiting for the platter to spin around until the required sector is under the head.
Seek time depends on the physical capabilities of the drive and is measured in milliseconds (ms). Latency depends on the rotational speed. The average access time is the combination of the average seek time and latency.
For example, a typical 7,200 RPM HDD has an average seek time around 8.5 ms and latency of 4.17 ms, so combined average access time is approximately 12.5 ms. This can significantly impact performance when doing lots of random I/O.
SSDs have virtually instantaneous access times of 0.1 ms or less, since they have no moving parts and use flash memory. This gives them up to 100x faster access times than HDDs.
In addition to maximum speeds and access times, real-world performance depends on a variety of factors:
- File sizes – Small random I/O vs large sequential I/O
- Queue depth – Multiple commands queued up
- Cache algorithms
- Workload – Read vs write intensive
- Fragmentation on HDDs
So while maximum speeds provide a general idea of performance, it’s important to look at benchmarks and reviews for comparisons of real-world results for specific workloads.
In summary, hard drive speeds are commonly measured in megabytes per second (MB/s) or megabits per second (Mbps). Key factors that affect speed include:
- Interface – SATA, SAS, USB, etc. determine maximum bandwidth
- Type – HDD vs SSD, with SSDs being much faster
- Spindle speed of HDDs – 7,200 RPM faster than 5,400 RPM
- Access times – SSDs have huge advantage over HDDs
- RAID configurations – can greatly increase speed
While exact speeds vary between models, typical values are:
- HDD: Up to 250 MB/s for high performance SATA models
- SATA SSD: Over 500 MB/s
- NVMe SSD: Over 3,000 MB/s
- External HDD over USB 3.0: Up to 120 MB/s
So in conclusion, newer interfaces like NVMe allow SSDs to reach speeds over 3 GB/s, while high performance HDDs are capped around 200-250 MB/s.