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

Question

Functionally, what's the difference between a USB flash drive and an SSD (aside from their interfaces, because duh)? Lots of nice gooey hardware discussion is totally welcome.

Answer 1

Mostly the interface. A USB flash drive uses USB, which is generally a slower interface than the Serial ATA interface. A USB flash drive is generally thought of as a removable device by the OS and is managed as such. A SSD is treated similarly to a hard drive, and is often recognized by your OS as a type of fixed disk.

USB drives tend to be optimized for size and cost, whereas SSDs are optimized more for speed and size, but this is not always the case.

Answer 2

USB flash drives and solid-state drives are both based on flash memory.

Most of the flash memory we use today are NAND memory, which are available as single-level cells (SLC) and multi-level cells (MLC).

Single-level cells store a single bit in a single memory cell, whereas multi-level cells store more than a single bit in a single memory cell. The MLC accomplishes this by allowing each memory cell to store multiple electrical states, therefore, allow one MLC cell to store more information than one SLC cell. For example, a MLC that uses 4-levels can store 2 bits of information.

Those differences account for the price difference of flash memory. MLC are cheaper, as they can store more information per memory cell than SLC. Cheap, large SSDs and USB flash memory use MLC disks, although not necessarily.

MLCs sounds great in terms of storage density, but it does have a catch -- MLCs are (generally) slower than SLC. Therefore, the large capacity USB flash drives and SSDs tend to be slower.

The difference of speed and capacity and its tradeoffs can be seen in SSDs used in netbooks. For example, my Eee PC 901 comes with two SSDs. The primary drive is SLC, and the secondary is MLC. The SLC drive is very fast, but has a capacity of only 4 GB, while the secondary is 8 GB, but very slow. I am guessing that a single large SLC-based SSD would have been too expensive to include in the low-cost netbook, Asus decided to include two, in order to account for the different uses -- primary drive for OS and applications, while secondary drive for data.

Another factor that affects the speed difference between flash memory and solid state drives are the drive controller for the flash drive. For example, the Intel SSDs which are known as fast SSDs, actually use MLC, but achieve very fast IO speeds. I believe I read somewhere that this is in part due to the drive controller being able to efficiently access the data on the drive.

Answer 3

Typical USB memory sticks use very slow chips, unless you buy a more expensive brand that is marketed for speed (I've had good luck with OCZ, the write speed difference was noticeable).

SSD drives are packed with very fast, very expensive chips.

Answer 4

As I understand it, in terms of how the data is physically stored and how the hardware is implemented there is little difference.

There are of course some changes in that the SSD has a cache and a large capacitor to keep the cache powered until the data is written to the disk if the power is suddenly lost. (because writes typically require a whole block to be written so the disk tends to delay writing until it can do a whole block or at least most of a block).

Additionally the charge pump mechanism in a SSD is smaller because it has access to slightly higher voltages than a USB device.

Beyond that I got nothing, though I am sure there are others who know other differences.

Answer 5

A very good USB thumb drive (I have one that is 30GB and was very expensive) actually boots Windows faster than my 250 GB 5400 rpm SATA2 drive.

With that being said, it was only marginally faster (30MB/s as measured) which shaved about 12 seconds off of the 45 seconds; when used in USB2.0 port (it was a 2.0 drive so 3.0 would have done nothing).

It was a lot slower than my new replacement drive (2 TB Seagate 7200 rpm SATA3 HDD), and I would imagine it being slower than any type of real SSD solution.

So I would expect a cheap USB drive of any kind to be slower than even a 5400 drive. Remember that USB 2.0 is capable of these high speeds (±150MB/s), and isn't the bottleneck. Buy an external SATA2 drive if you need proof.

• My cheap USB: read 20-30M/s (a 4GB usb drive I have lying around)
• My USB drive: read 97M/s (this one was the expensive 30 GB thumb drive)
• My old 5400 : read 71M/s
• My new 7200: read 128M/s

A good SATA2 SSD can hit 250-280M/s.