What are the three differences between an HDD and an SSD?

There are a few key differences between traditional hard disk drives (HDDs) and solid state drives (SSDs) when it comes to computer storage. The three main differences are:

1. Storage medium

HDDs use magnetic disks to store data. They have platters that spin at high speeds with read/write heads floating just above them to access data. SSDs use flash memory chips to store data with no moving parts. Flash memory relies on electrons rather than magnetics for data storage.

2. Speed

SSDs are much faster than HDDs in most use cases because they can access data almost instantly. HDDs require the read/write head to physically move to the part of the disk where the data is stored, which takes longer. Typical SSD speeds are over 100 MB/s for reads and writes while HDDs max out around 160 MB/s for sequential reads and 80-120 MB/s for writes.

Drive Type Typical Read Speed Typical Write Speed
SSD Over 100 MB/s Over 100 MB/s
HDD Up to 160 MB/s 80-120 MB/s

3. Reliability

SSDs are generally more reliable than HDDs because they have no moving parts. HDDs with spinning disks are susceptible to damage from drops, vibration, magnetism, etc. That said, SSDs can still fail over time as cells wear out from repeated erase/write cycles. HDDs are susceptible to mechanical failure over time too.

Overall SSDs tend to last longer than HDDs in most use cases. One reliability measure is annualized failure rate (AFR) which measures percentage of drives failing in a year. Typical AFR for consumer HDDs is around 1-2% compared to 0.5-1% for consumer SSDs.

Drive Type Typical Annual Failure Rate
HDD 1-2%
SSD 0.5-1%

In summary, the three key differences between HDDs and SSDs are:

  • HDDs use magnetic disks while SSDs use flash memory chips
  • SSDs are much faster for most workloads
  • SSDs tend to be more reliable with no moving parts

When choosing storage, consider capacity needs, performance requirements, and reliability expectations. HDDs offer more capacity per dollar while SSDs provide faster speeds, better reliability, and resistance to shock and vibration.


The ongoing competition between hard disk drives (HDDs) and solid state drives (SSDs) is driving innovation and improvements in computer storage technology. HDDs still offer the best value for high capacity bulk storage needs. But for applications requiring fast access like operating systems and programs, SSDs are now the standard.

SSD prices continue to fall, making them viable even for large multi-terabyte storage needs. At the same time, HDD capacities and prices per gigabyte continue to improve. There is a place for both technologies in most computing systems. Understanding the key differences in how they work and perform allows selecting the best storage technology for each application.