Two ways to keep data without power
Both kinds of drive do the same job. They keep files, apps and the operating system when the computer is off. The difference lies in how the bits are held. A hard disk drive (HDD) stores them magnetically on spinning disks. A solid state drive (SSD) stores them electrically in flash memory chips with no moving parts.
How a hard drive works
Inside a hard drive are one or more rigid disks called platters, coated with a magnetic material. They spin at a steady speed, commonly 5,400 or 7,200 revolutions per minute in consumer drives. A small arm moves a read-write head across the surface, hovering extremely close to it without touching. The head records data by changing the magnetic direction of tiny regions of the platter, and reads it by sensing those directions.
Because the head must physically move to the right place and then wait for the right part of the disk to spin underneath it, there is a small delay each time data is requested. That delay is short by human standards, but it adds up. It is especially noticeable when a program needs many small pieces of data scattered around the disk, as happens when a computer starts up.
How an SSD works
An SSD stores each bit in a tiny cell in a flash memory chip, as an electrical charge held in a way that persists without power. A controller chip manages reading, writing and keeping track of where everything is. There is no spinning and no moving head, so any location can be reached almost immediately.
Flash has a quirk. Data is written in small units but can only be erased in larger blocks, and each cell tolerates a limited number of write-and-erase cycles before it wears out. The controller spreads writes evenly across the chips, a technique known as wear levelling, and keeps spare capacity in reserve. For ordinary use this wear is rarely a practical concern during the normal life of a computer, and drive makers often publish an endurance rating if you want to check.
Speed
SSDs are generally much faster than hard drives, especially for starting the system, opening programs and handling lots of small files. The gap in everyday feel is often large: a computer that took a long time to start with a hard drive may start far more quickly after switching to an SSD. Exact numbers vary widely between models and connection types, so treat any single figure with caution.
Speed also depends on how the drive connects to the computer. Common SSDs use the same kind of connection as hard drives, while newer ones plug in more directly and can be faster still. Hard drives have a lower limit however they are connected, because the mechanics are the bottleneck.
Durability
Moving parts are the main weakness of hard drives. A knock or drop while the disk is spinning can damage the platters or the head. That is a particular concern for laptops and portable drives. Hard drives can also fail gradually through mechanical wear, sometimes with warning signs such as clicking noises or slow, error-prone access.
SSDs handle shocks better because they have nothing to knock out of place. They are not immune to failure, though. Chips and controllers can fail, sometimes suddenly with little warning, and flash does wear with heavy writing. In practice neither type is permanently reliable, which is why backups matter whichever you use. Losing important files when a drive fails is a risk with both, and the details of data recovery are different for each.
Capacity and cost
Hard drives generally cost less per gigabyte, particularly at large sizes of several terabytes. That keeps them useful for bulk storage such as large media collections and archives. SSD prices have fallen over the years and they are now the usual choice for the main drive in most new computers, but for the same money you will normally get less space than with a hard drive. Prices change over time, so check current listings rather than relying on a fixed figure.
Noise, heat and power
Hard drives make audible sounds from spinning and seeking, and they use more power. SSDs are silent and typically use less power, which can help laptop battery life a little, although the effect is usually modest next to the screen and processor. Some very fast SSDs can get warm under sustained load, and may slow themselves down to control heat.
A quick comparison
| SSD | Hard drive | |
|---|---|---|
| Storage method | Flash memory chips | Magnetic spinning platters |
| Speed | Fast, especially for small files | Slower, with mechanical delays |
| Moving parts | None | Yes |
| Shock resistance | Better | Weaker, especially while running |
| Cost per gigabyte | Higher | Lower |
| Noise | Silent | Audible |
| Typical use | System drive, laptops, phones | Bulk storage, backups, archives |
Which to choose
For a main drive that holds the operating system and your programs, an SSD is usually the better choice because of the speed and robustness. Replacing a hard drive with an SSD is often described as one of the more noticeable upgrades for an older computer, if it supports it, though results depend on what was slowing the machine in the first place.
A hard drive can still make sense for large collections of files that do not need fast access, or as an inexpensive place for backups. Many desktops use both: a smaller SSD for the system and a larger hard drive for storage.
Phones, tablets and many thin laptops use soldered flash storage, so there is no choice to make, and the amount you buy at the start is what you will have. Phone storage works on the same flash principles described here.
Whichever you pick, it helps not to fill a drive completely, and clearing disk space before it runs out is easier than fixing a full drive. An SSD in particular works better with some free space available, and a nearly full drive of either type tends to be slower and harder to manage.