Flash Storage Guide

Learning Center › Flash Storage

Learning Center

Flash Storage

How eMMC, UFS, NAND flash, SSDs and USB drives work, how to read speed and capacity numbers, and where each one fits.

Basics →eMMC and UFS →NAND Flash →SSDs →USB →

eMMCEmbedded storage

NAND FlashCore technology

SSDsSolid state drives

USBPortable storage

Overview

Different flash for different jobs.

Flash memory is in almost every device you use. Each form solves a different problem: some put speed first, some cost, some fitting into tiny spaces. Here is what each major type is, how it works and where you will find it.

At a glance

One chip, many packages.

USB drives, SD cards, SSDs, eMMC and UFS all store data on NAND flash, which keeps data with the power off. Each also has a controller: a small processor that decides where data goes, spreads wear evenly, fixes small errors and talks to the device. What makes them different products is mostly the package and the connection. Memory (RAM) is the exception: it uses DRAM, which is far faster but forgets everything when the power goes off.

Product	Memory	Connects through	Typical speed (approx.)
USB flash drive	NAND flash	USB port	10 to 400 MB/s
SD / microSD card	NAND flash	Card slot	10 to 300 MB/s (SD Express is faster)
Portable SSD	NAND flash	USB or USB-C cable	400 to 2,000 MB/s
Internal SSD	NAND flash	SATA cable or M.2 slot	550 MB/s (SATA) to 14,000 MB/s (PCIe Gen5)
eMMC	NAND flash	Soldered to the board	Up to 400 MB/s
UFS	NAND flash	Soldered to the board	1.2 to 5.8 GB/s
Memory (RAM)	DRAM	DIMM or SO-DIMM slot, or soldered	10 to 50+ GB/s per module

Reading speed numbers

Bits, bytes and "up to".

Connection speeds (USB, SATA, PCIe) are listed in bits per second, Mbps or Gbps with a small "b". Product speeds on the package are in bytes per second, MB/s or GB/s with a capital "B". There are 8 bits in a byte, so divide by 8 to compare. Real-world speed is always lower, because part of every connection carries error checking and control signals.

Connection	Rated speed	Divided by 8	Real-world best (approx.)
USB 2.0	480 Mbps	60 MB/s	35 to 40 MB/s
USB 3.2 Gen 1 (USB 3.0)	5 Gbps	625 MB/s	about 450 MB/s
USB 3.2 Gen 2	10 Gbps	1,250 MB/s	about 1,000 MB/s
SATA III	6 Gbps	750 MB/s	about 550 MB/s

The connection is the speed limit, not the speed. How fast a product really goes depends on the flash and controller inside it. Keep these in mind when reading a spec sheet:

"Up to" means best case, usually measured copying one large file.
Read and write speeds differ. Reading is almost always faster. If copying files onto a drive feels slow, write speed is the number that matters.
Sequential vs random. Sequential is one big file, like a movie. Random is many small files at once, measured in IOPS (input/output operations per second); it is what running apps feels like.
The slowest link wins. A fast drive in a slow port runs at the port's speed, and a slow drive in a fast port runs at the drive's speed.

Capacity

What the numbers hold.

Capacity	Phone photos (approx.)	Full HD video	4K video
32 GB	8,000	6 hours	1.5 hours
128 GB	32,000	25 hours	6 hours
512 GB	128,000	100 hours	25 hours
1 TB	250,000	200 hours	50 hours

Estimates assume about 4 MB per photo, 5 GB per hour of Full HD video and 20 GB per hour of 4K phone video. Professional cameras make much larger files. Why does a 128 GB drive show 119 GB? Makers count 1 GB as 1,000,000,000 bytes and computers count 1,073,741,824, so the same bytes show as a smaller number. 1 TB shows as about 931 GB.

Embedded storage

eMMC

Short for "embedded MultiMediaCard." A compact package that combines flash memory and its controller on a single chip, for devices where small size and low power matter more than raw speed.

How it works. The flash and the controller that manages it sit in one package and talk to the device's main processor through the MMC interface. That integration makes eMMC small enough for phones, tablets and dashboards. Device makers do not need to design their own flash controller or manage low-level memory operations; the package handles it all.

Where you will find it:

Smartphones and tablets, especially budget and mid-range models
Digital cameras and handheld devices
Automotive systems: navigation, infotainment, driver assistance
Industrial devices: medical equipment, factory automation, surveillance cameras

What's next: UFS. In higher-end devices, UFS (Universal Flash Storage) has become the successor, with faster transfers over a different interface. eMMC remains the go-to for cost-sensitive, space-constrained applications, and will be for a long time.

Embedded storage

eMMC vs UFS

Both are flash plus a controller in one chip, soldered to the board. eMMC is a one-lane bridge: it reads or writes, one direction at a time. UFS is a two-lane road: it reads and writes at the same time, over faster lanes. A board designed for one cannot take the other, and the device's processor decides which UFS generation it supports.

	eMMC	UFS
Full name	embedded MultiMediaCard	Universal Flash Storage
Data flow	Read or write, one at a time	Read and write at the same time
Top speed	About 400 MB/s (eMMC 5.1, HS400)	About 3x to 15x faster, by generation
Typical use	Budget and mid-range devices, Chromebooks, automotive, industrial and IoT	Mid-range and flagship phones, newer automotive, high-performance embedded

eMMC details

Versions and packages.

Versions

eMMC 4.5 / 4.51: HS200 mode, about 200 MB/s, found in older designs.
eMMC 5.0: HS400 mode, about 400 MB/s.
eMMC 5.1: HS400 plus command queuing for faster small-file work. The current standard. Newer eMMC generally runs in a design built for an older version, at the older mode.

153-ball vs 100-ball

eMMC is a BGA (ball grid array) chip. 153-ball is 11.5 x 13 mm at 0.5 mm pitch, the standard footprint for space-constrained designs. 100-ball is 14 x 18 mm at 1.0 mm pitch, used in industrial and automotive boards where larger solder joints matter. Same interface; the difference is fit, not speed. UFS often uses a similar-looking 153-ball package but is not interchangeable.

UFS generations

Each roughly doubles the last.

Generation	Top speed (approx.)	Where you will find it
UFS 2.1	1.2 GB/s, about 3x eMMC	Budget phones and tablets, many embedded and industrial designs
UFS 3.1	2.9 GB/s	Mid-range phones and newer automotive systems
UFS 4.1	5.8 GB/s	Flagship phones and high-performance systems

These are interface maximums. Real read and write figures depend on the part, capacity and NAND type, so use the datasheet for the exact part.

Designing it in

Temperature grades and lifecycle.

-25°C to +85°C

Commercial Extended

Consumer and indoor devices.

-40°C to +85°C

Industrial

Factory, medical, outdoor and surveillance equipment.

-40°C to +105°C

Automotive Grade 2

In-vehicle systems such as infotainment and driver assistance.

Built for a long product life. Embedded designs are often built for 5 to 10 years or more. Engineers qualify one part number, so they need it to stay available and unchanged: a fixed bill of materials, a PCN (Product Change Notification) before any change and an EOL (End of Life) notice before a part is discontinued. See BOM Control for how Optima and Optima+ handle this.

Core technology

NAND Flash

Non-volatile storage: it keeps your data when the power is off. NAND flash is inside your phone, SSD, USB drive, memory card, car, TV and every other device that remembers files without a battery.

NAND types differ in how many bits each memory cell holds. Fewer bits per cell means faster writes, longer life, lower capacity and higher cost per GB. More bits per cell means cheaper, bigger drives, but slower writes and a shorter lifespan.

Types of NAND flash

Bits per cell.

1 bit / cell

SLC (Single-Level)

Fastest, most durable, lowest capacity, highest cost. Used in enterprise and industrial applications.

1 bit / cell, in SLC mode

pSLC (pseudo-SLC)

MLC or TLC flash run one bit per cell. Far higher endurance than TLC, at the cost of capacity. Used in industrial eMMC and SSDs.

2 bits / cell

MLC (Multi-Level)

Balanced performance and capacity. Common in older consumer SSDs and higher-endurance uses.

3 bits / cell

TLC (Triple-Level)

The standard for most consumer SSDs today. A good balance of speed, cost and capacity.

4 bits / cell

QLC (Quad-Level)

Highest capacity at a lower cost per GB, with lower endurance. Common in high-capacity consumer SSDs.

5 bits / cell

PLC (Penta-Level)

Emerging technology. Pushes capacity further at the cost of speed and endurance.

3D NAND stacks cells in layers, like floors in a building, to fit more capacity on a chip. It is the norm today and not a separate cell type: 3D TLC still stores three bits per cell.

Endurance and grade

Why it matters.

Endurance

Each cell can be erased and rewritten only a limited number of times, called Program/Erase (P/E) cycles. SLC handles roughly 100,000 cycles; QLC around 1,000. For everyday use you will replace the device first. For heavy or industrial workloads, endurance is the deciding factor. SSDs list it as TBW (terabytes written), the total data the drive is rated to take; a 1 TB consumer TLC drive is typically rated in the hundreds of TBW.

Industrial vs consumer grade

Industrial NAND is built for extended temperatures, higher endurance and longer support lifecycles, so a device built today can be built identically years from now. Consumer NAND optimizes for cost and capacity.

Solid state drives

SSDs

A solid state drive stores data on NAND flash chips instead of spinning magnetic platters. No moving parts means faster access, better reliability, lower power draw and silent operation.

Form factors

Four common shapes.

Traditional

2.5 inch

The classic laptop drive size. Fits laptop drive bays, or desktops with a 3.5" adapter. Usually SATA.

Modern

M.2

A slim stick that plugs straight into the motherboard. Can be SATA or NVMe; sizes and keys below.

Legacy desktop

PCIe add-in card

A full-size card for older desktop motherboards without M.2 slots. Still offers full NVMe speeds.

External

Portable SSD

A small box on a USB or USB-C cable. No tools or installation, and it works with any computer, so it is the safe choice when you are not sure what is inside.

Interfaces

SATA vs NVMe

Interface	Best for	Typical max speed
SATA	Older systems, budget builds, everyday use where raw speed is not critical	~550 MB/s
NVMe (PCIe 3.0)	Modern laptops and desktops, gaming, content creation	~3,500 MB/s
NVMe (PCIe 4.0)	Latest systems, high-end workloads, large file transfers	~7,000 MB/s
NVMe (PCIe 5.0)	Cutting-edge systems, professional workloads	~14,000 MB/s

Pro tip: match the drive to your motherboard. An NVMe drive will not work in a SATA-only slot, and a PCIe 4.0 drive on a PCIe 3.0 motherboard runs at 3.0 speeds. Check which interface your system supports before buying.

M.2 sizes and keys

Check the slot before you buy.

The code on an M.2 drive gives its size: the first two digits are the width and the rest the length, in millimeters. A longer drive never fits a shorter slot.

Size	Dimensions	Where you will find it
2230	22 x 30 mm	Ultra-thin laptops, mini PCs, handheld gaming PCs
2242	22 x 42 mm	Compact and business laptops, industrial and embedded PCs
2280	22 x 80 mm	Most laptops and desktops, by far the most common
22110	22 x 110 mm	Servers and workstations, with room for power-loss protection

The notches in the gold edge are called keys, and they are the quickest clue to the drive type:

Key	Looks like	Usually means
M-key	One notch near one edge	PCIe NVMe (x4), nearly all current NVMe SSDs
B-key	One notch near the other edge	SATA or PCIe x2; rare on its own today
B+M key	Two notches	Almost always M.2 SATA

The trap: a B+M SATA drive physically fits an M-key slot, but if that slot supports only NVMe the computer will not see it. Fitting is not the same as working; check whether the slot supports SATA, NVMe or both.

Other SSD specs

What else is on the spec sheet.

PCIe lanes and NVMe

PCIe connections are built from lanes; most M.2 NVMe drives use four (x4), and budget drives using two (x2) top out at about half the speed. NVMe is the language the drive speaks over PCIe, designed for flash, so "PCIe SSD" and "NVMe SSD" mean the same thing. PCIe generations work both ways: a Gen5 drive runs in a Gen3 slot at Gen3 speed.

DRAM vs DRAM-less

Many SSDs carry a small DRAM chip as a fast map of where data lives, which keeps speed steady under heavy use. DRAM-less drives cost less; NVMe versions borrow a little of the computer's memory instead (HMB, Host Memory Buffer). Fine for everyday use.

SLC cache: why drives slow down

Most consumer SSDs write first to a fast cache. During a very large copy, once the cache fills, write speed drops, sometimes sharply on QLC and DRAM-less drives. That explains "it started fast, then slowed down".

Portable SSD speed

A portable SSD can only go as fast as its USB link. On a 10 Gbps connection it tops out around 1,000 MB/s, and only in a 10 Gbps port.

Upgrading

Should you upgrade to an SSD?

If you still run a spinning hard drive: yes, without hesitation. An SSD is the single biggest performance upgrade you can make to a computer. Boot times, app launches and file loads all get noticeably faster, and even a budget SATA SSD feels dramatically faster than a hard drive.

Portable storage

USB Technology

The USB flash drive is still one of the most popular portable storage devices in the world: small, inexpensive, fast and plug-and-play everywhere.

What's inside. A USB connector, one or more NAND flash chips, a controller that manages reads and writes, and a crystal oscillator for timing, all in a shell small enough for a keychain.

USB generations

Generations and speeds.

Current name	Also sold as	Connection speed	Connector
USB 2.0	Hi-Speed USB	480 Mbps	Type-A or Type-C
USB 3.2 Gen 1	USB 3.0, USB 3.1 Gen 1, "USB 5Gbps"	5 Gbps	Type-A or Type-C
USB 3.2 Gen 2	USB 3.1 Gen 2, "USB 10Gbps"	10 Gbps	Type-A or Type-C
USB 3.2 Gen 2x2	"USB 20Gbps"	20 Gbps	Type-C only
USB4	"USB 40Gbps" or "USB 80Gbps" (USB4 Version 2.0)	40 or 80 Gbps	Type-C only

The same speed has been renamed several times, so one drive may be sold as USB 3.0, USB 3.1 Gen 1 or USB 3.2 Gen 1. A blue tab inside a Type-A plug often marks USB 3.x, but the color is optional; check the package. Most flash drives are held back by the chips inside long before they reach the connection's limit, especially when writing. Portable SSDs are the products that come close.

Backward compatible. A USB 3.2 drive works in a USB 2.0 port, at USB 2.0 speeds.

Connectors

Type-A vs Type-C

Classic

Type-A

The rectangular connector that has been around for decades. One orientation only. Still the most common port on desktops and older laptops.

Modern

Type-C

Smaller and reversible. Standard on newer laptops, phones and tablets.

USB-C is a shape, not a speed. A USB-C port or drive can be anything from USB 2.0 to USB4, and many phones have USB-C ports that run at USB 2.0 speed. The connector tells you what fits; the USB version tells you how fast it goes. Many flash drives now have both connectors, one on each end, which is handy for moving files between old and new devices.

Beyond storage