Hardware · Topic 5 of 13
Storage Devices
Compare hard disk drives, solid state drives, NVMe and M.2, SATA, flash media and optical discs by speed, cost, capacity and typical use in PCs.
- 4 min
- HDDs have spinning platters; 5400 and 7200 rpm are common speeds.
- SSDs use flash memory, with no moving parts, and are faster and quieter.
- 3.5 inch drives are for desktops and 2.5 inch drives for laptops.
What it is
Storage keeps programs and files when the power is off. Different devices trade speed, capacity, cost and durability, so choosing one is a matter of matching it to the job.
The main families are hard disk drives, solid state drives, removable flash media and optical discs. Redundant array of independent disks (RAID), a way to combine drives, is covered on the next page.
How it works
Hard disk drives
A hard disk drive (HDD) stores data on spinning magnetic platters, read by heads on a moving arm. It is mechanical, so it is slower, makes noise and can fail from knocks.
| Feature | Detail |
|---|---|
| Sizes | 3.5 inch for desktops and 2.5 inch for laptops and compact PCs |
| Speed | Rated in revolutions per minute (rpm): 5400 and 7200 are common, 10,000 and 15,000 are used in servers |
| Strength | Low cost per gigabyte, large capacity |
| Weakness | Slow access, noise, heat, sensitive to shock |
A faster rpm lowers the wait for data but raises noise, heat and power use.
Solid state drives
A solid state drive (SSD) stores data in flash memory chips and has no moving parts. It is faster, silent, uses less power and handles shocks well. It costs more per gigabyte than an HDD.
SSDs wear out after a large amount of writing, but normal use rarely reaches that limit. A hybrid drive (SSHD) pairs a small flash cache with a larger spinning disk.
Interfaces and form factors
| Type | Interface | Notes |
|---|---|---|
| 2.5 inch SATA SSD | Serial Advanced Technology Attachment (SATA) | Same shape as a laptop hard drive; up to about 550 megabytes (MB) per second in practice |
| M.2 SATA | SATA through a card slot | Small stick; same speed as a SATA drive |
| M.2 NVMe | Peripheral Component Interconnect Express (PCIe) | Much faster than SATA, using the Non Volatile Memory Express (NVMe) protocol |
| mSATA | SATA through a mini card slot | Older small form factor |
M.2 is the physical card shape, not the speed. An M.2 slot may support SATA, NVMe or both, so check the board manual. The card is held by a single screw at one end.
NVMe drives talk over PCIe lanes instead of the older SATA link, which is why they are so much faster.
Serial Attached SCSI (SAS) is a serial interface used in servers and storage arrays. It offers higher speeds and better reliability than SATA, and its drives are fast, often spinning at 10,000 or 15,000 rpm. A SAS controller can run both SAS and SATA drives, but a plain SATA port cannot run a SAS drive.
Flash media
Flash media include universal serial bus (USB) flash drives and memory cards such as Secure Digital (SD) and microSD. They are small, portable and fine for moving files or extending phones and cameras. They are not meant as long term, heavy use storage.
Optical drives
Optical drives read discs with a laser. They are slow, but discs are cheap and handy for installation media and archives.
| Disc | Typical capacity |
|---|---|
| Compact disc (CD) | About 700 MB |
| Digital versatile disc (DVD) | 4.7 gigabytes (GB) per layer |
| Blu-ray | 25 GB per layer |
Recordable discs (R) can be written once, and rewritable discs (RW) can be erased and written again.
Choosing storage
When a scenario asks for the best performance and the board has an M.2 slot for NVMe, choose an NVMe M.2 SSD. When it asks for the cheapest large capacity, choose an HDD.
Preparing a new drive
A new drive must be initialised and formatted with a file system before it can store files. Operating system tools do this. The drive then gets a drive letter or mount point.
What to remember for the exam
- HDDs have spinning platters; 5400 and 7200 rpm are common speeds.
- SSDs use flash memory, with no moving parts, and are faster and quieter.
- 3.5 inch drives are for desktops and 2.5 inch drives for laptops.
- SATA is the common interface; NVMe over PCIe is much faster.
- M.2 is a form factor; the card may be SATA or NVMe.
- A hybrid drive combines flash cache with a spinning disk.
- CD is about 700 MB, DVD 4.7 GB and Blu-ray 25 GB per layer.
- A new drive needs to be initialised and formatted before use.
Common mistakes
Assuming M.2 always means NVMe. M.2 describes the shape; the interface may be SATA.
Buying NVMe for a board that has no support. Check the slot type before buying.
Choosing an HDD for a laptop that moves a lot. Knocks can damage spinning disks.
Treating an SSD like it lasts forever. Flash cells wear out, but typical home use rarely wears a drive out quickly.
Mixing up 3.5 inch and 2.5 inch bays. A 2.5 inch drive in a desktop needs a bracket.
Check yourself
Which storage type has no moving parts?
A solid state drive (SSD).
What interface lets M.2 drives reach much higher speeds than SATA?
NVMe over PCIe.
Which disc type holds about 25 GB per layer?
Blu-ray.
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