What's the technical difference between a flash drive and an SSD?

Both Flash and SSD are based on NAND-based flash memory, which retains data without power, and so can be labelled as Flash memory.

Technologically, the main differences between the two are in :

1. The underlying technology used to construct the NAND,
2. The quality of the Flash memory controller,
3. The computer connector : USB or SATA.

NAND technologies

NAND technologies diverge on two points : Speed and price.

On the one hand one finds MLC (Multi-level cell) which is a memory element capable of storing more than a single bit of information. Most MLC NAND flash memory has four possible states per cell (or even more with TLC), so it can store several bits of information per cell. This reduces the number of required transistors, so reducing size and manufacturing costs, while also reducing speed and increasing the possibility of errors.

On the other hand one finds SLC (single-level cell), where each cell can exist in one of two states, storing one bit of information per cell. This increases the access speed, while also increasing manufacturing costs and electricity usage.

An MLC cell is typically rated at 10,000 erase/write cycles, while an SLC cell might last 10 times that before failing.

Because of these differences, MLC is typically used in slower and cheaper media, accessed typically via USB. A good SSD will use SLC and be costlier, but faster, have a longer life-time and be typically accessed via SATA 2 or 3.

Memory controller

A USB mass storage controller has only a small micro-controller with a small amount of on-chip ROM and RAM.

An SSD controller is much more complicated. The controller is an embedded processor that executes firmware-level code and is one of the most important factors of SSD performance. Some of the functions performed by the controller include:

• Error-correcting code (ECC)
• Wear leveling
• Bad block mapping
• Read scrubbing and read disturb management
• Read and write caching
• Garbage collection
• Encryption

In a hybrid SSD, the controller will also manage a small classical hard disk.

Connector

A flash stick normally uses a standard-A USB plug that provides the physical interface to the host computer. These can now go up to USB-3 speeds for the more costly models, or USB-2 for the common ones.

SSD technology uses electronic interfaces compatible with traditional block input/output (I/O) internal hard disk drives. Additionally, new I/O interfaces, like SATA Express, have been designed to address specific requirements of the SSD technology. Most SSD cards are typically much faster than classical hard drives.

Summary

A Flash stick typically has less memory capacity, is slower, cheaper and is also less dependable than an SSD.

There are of course always devices that bridge these differences by using compensating technologies.

References :

• Wikipedia Solid-state drive
• Wikipedia USB flash drive
• Wikipedia Multi-level cell (MLC)
• Wikipedia Flash memory
• Micron TLC MLC and SLC Devices
• MLC vs SLC: Which flash SSD is right for you