A hard disk drive (HDD) is a data storage device that uses magnetic storage to store and retrieve digital data. Most HDDs have multiple spinning magnetic disks, also called platters, that are paired with magnetic heads arranged on moving actuator arms. The platters and heads form a disk stack, and the number of platters determines the total storage capacity of the hard drive. So how many disks are typically found inside a hard drive?
Typical Number of Disks in a Hard Drive
Most modern hard drives have either 1, 2, 3 or 4 disks (platters). Some key points:
- Smaller hard drives designed for desktops and laptops often have just 1 or 2 disks.
- Standard desktop hard drives typically have 2 or 3 disks.
- Larger high-capacity hard drives can have up to 4 or more disks.
- Enterprise and server hard drives designed for heavy workloads may have up to 8 disks.
So in general, the more storage capacity a hard drive has, the more disks it will have inside.
1 Disk Hard Drives
Small form factor hard drives designed for space-constrained applications like laptops and tablets frequently have just a single disk inside. For example, many 2.5″ laptop hard drives have a single platter that provides 500GB to 2TB of storage. Having a single disk allows these drives to be very thin and compact.
2 Disk Hard Drives
Most desktop hard drives have 2 or 3 disks stacked vertically to provide ample storage capacity. 2 disk drives are very common for desktop computers, providing 500GB to 4TB of space. The two spinning platters are affixed to a common spindle and rotate in unison to achieve fast data access times.
3 Disk Hard Drives
Many standard 3.5″ desktop hard drives have 3 vertically stacked disks. Three disks allow these drives to offer high capacities from 1TB up to 10TB or more. Popular models like the Seagate Barracuda Compute often come equipped with 3 disks to offer ample storage for desktop PCs.
4 Disk Hard Drives
Higher capacity 3.5″ hard drives often have 4 internally stacked disks. This allows them to reach huge storage capacities from 10TB to 16TB or more. For example, the massive Seagate IronWolf Pro 16TB drive uses 4 disks to offer enterprise-levels of storage. The extra disk density provides tremendous storage but also generates more heat.
Larger Disk Counts in Enterprise Drives
While consumer hard drives top out at 4 disks, enterprise and server-class hard drives designed for data centers may incorporate up to 8 or more disks to achieve massive capacities.
For example, Seagate’s Exos X18 rackmount hard drive offers a staggering 18TB of space using 9 disks in a 3.5″ form factor. These types of drives are not intended for typical desktop use but are built for heavy duty 24/7 operation in high-demand environments.
How Disks Provide More Capacity
Adding more disks to a drive increases the total storage capacity for a few key reasons:
- More recording surfaces – Each disk provides two recordable surfaces, one on the top and another on the bottom. More disks means more surfaces available to store data.
- Increased areal density – More disks allow the drive heads to access more square footage of magnetic recording area inside the drive.
- Improved data access – With more disks, reads and writes can be split between more heads working in parallel to deliver higher performance.
However, more disks also increase manufacturing costs, power consumption, heat output, and potential points of failure. So drive engineers must strike an optimal balance between capacity, performance and reliability when designing multi-disk HDDs.
Common Disk Capacities
Assuming typical areal densities, here are some approximate per-disk storage capacities for modern hard drives:
- 2.5″ HDD disks – Up to 1TB per disk
- 3.5″ HDD disks – Up to 4TB per disk
- Enterprise 3.5″ disks – Up to 8TB per disk
Of course, areal density improvements allow drive manufacturers to squeeze more capacity from each disk over time. But the per-disk numbers above represent a general guideline for common models today.
How Many Platters Do SSDs Have?
Solid state drives (SSDs) use flash memory chips rather than magnetic disks to store data. So technically SSDs do not have “platters” or spinning magnetic disks inside them. However, some SSDs are designed to physically fit into the same 2.5″ or 3.5″ hard drive form factors and may include empty spacer disks to fill this space.
In general SSDs have vastly simpler internal constructions than hard drives, with no moving parts. But they achieve incredible speeds by using stacked flash memory chips mounted on compact circuit boards.
Drive Interface Types
While the drive enclosure and interior disks make up the physical storage, the drive interface is how the HDD electronically connects to the computer or controller. Here are some common hard drive interface types:
SATA Hard Drives
Serial ATA or SATA is the most popular hard drive interface for consumer and entry-level enterprise use. SATA drives feature thin cables up to 1 meter long and transfer speeds up to 6Gbps.
SAS Hard Drives
Serial Attached SCSI or SAS hard drives feature a higher-end serial interface designed for enterprise and data centers. SAS supports multiple drives on a single interface and achieves speeds over 12Gbps.
NVMe or Non-Volatile Memory Express is a high-speed PCIe host interface designed for SSDs. NVMe SSDs offer incredible performance with transfer rates exceeding 3.5GB/s.
Comparing Drive Physical Size Factors
Hard drives and SSDs come in a variety of physical form factors. The size and shape of the drive housing is known as its physical format. Common formats include:
The standard 3.5″ desktop hard drive size with heights from 1″ to 1.6″. 3.5″ drives provide expansive capacities from 500GB to 16TB+. Most have SATA, SAS or NVMe interfaces.
Slim 2.5″ form factor used in laptops and enterprise arrays. 2.5″ HDDs range from 320GB to 2TB typically. High performance 2.5″ SSDs are popular in gaming laptops and servers.
Tiny 1.8″ hard drives designed for small portable devices. Extremely compact but limited to around 120GB. Being phased out in favor of flash storage.
M.2 SSDs designed to mount directly to motherboard M.2 slots. Very compact and offers blazing NVMe speeds up to 4000MB/s.
U.2 uses the 2.5″ HDD form factor but with a high-speed PCIe NVMe interface. Achieves similar speed to M.2 but uses standard hot-swap bays.
Comparing Common Hard Drive Sizes
This table compares the typical attributes of popular hard drive size formats:
|3.5″ HDD||4″||1″||5.8″||SATA, SAS||500GB – 16TB|
|2.5″ HDD||2.8″||0.4″||3.9″||SATA, SAS||320GB – 2TB|
|M.2 SSD||0.8″||0.1″||2.4″||PCIe NVMe||128GB – 2TB|
|1.8″ HDD||1.9″||0.2″||2.1″||IDE, SATA||40GB – 120GB|
Choosing the Right Drive for You
When selecting a new hard drive or SSD, consider factors like:
- Capacity – How much storage space do you need today and in the future?
- Performance – Will a faster SSD or enterprise HDD impact your workflows?
- Form factor – Does it need to fit specific size requirements?
- Interface – Select the right interface to match your system.
- Data security -Enterprise drives offer more features to protect data.
- Warranty – Look for at least 2-3 years of warranty coverage.
- Price – How much does capacity, speed and form factor influence price?
Matching your needs to the right storage technology and form factor will help ensure you get the perfect hard disk or SSD for your use case.
In summary, most hard drives have between 1 to 4 spinning magnetic disks inside that store and read data. Smaller 2.5″ HDDs have 1-2 disks, while larger 3.5″ desktop drives typically have 2 or 3 disks, and high capacity drives may have up to 4 disks. More disks allow for greater storage capacity thanks to increased recording surfaces and areal density. Enterprise hard drives designed for data centers may have many more disks to achieve massive capacities. So the number of disks directly influences the total storage potential for a hard drive.