Not all SSDs are the same. Some are enterprise class and some are at various levels for the consumer, trading speed for size and cost.

The other day, I was chatting with John Kheit, one of our TMO authors, about SSDs. He quickly dropped into the nomenclature of layers, something I was only vaguely aware of. So I did some research an found an excellent introductory article.

Author Ian Paul discusses the five types of SSD, the origin of the three-letter initialism, and the pros and cons. Here are the five types from Wikipedia (which itself goes into a bit more detail).

  1. SLC (1 bit per cell) – fastest, highest cost
  2. MLC (2 bits per cell)
  3. TLC (3 bits per cell)
  4. QLC (4 bits per cell) – slowest, lowest cost
  5. PLC (5 bits per cell) – currently in development

Author Paul has this to say about TLC.

As its name implies, TLC SSDs write three bits to each cell. At this writing, TLCs are the most common type of SSD.

They pack more capacity than SLC and MLC drives into a smaller package, but sacrifice relative speed, reliability, and durability. That doesn’t mean TLC drives are bad. In fact, they’re probably your best bet right now—especially if you’re hunting for a deal.

Don’t let the notion of less durability get you down; TLC SSDs usually last for several years.

OWC Envoy Pro

OWC Envoy Pro EX

I’ve inquired with Other World Computing on which type they use in the Envoy Pro EX. I’ll update this article when they let me know.

Finally, author Paul concludes with a nifty glossary of SSD terms. Terms like 3D NAND, wear leveling, and NVMe are briefly defined. Now that we’re all booting from SSD Macs and starting to use SSDs as Time Machine drives, this nomenclature is good to have handy at our mental finger tips.

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Thanks John. I just restored my backup to the new OWC 1TB SSD from them today. My old 250Gb was too small for my 2012 MacBook Pro. Now I’ll have to check the type of the drive.


And Octa-Level (OLC) NAND.


Quite timely since I’m just in the process of recovering what I can from a 2 1/2-year-old SSD.

Not a fun experience.