The author wrongly dismisses the impact of the SSD's spare area on usable storage:
> "The parts about wear-leveling blacks and bad blocks are just part of how SSDs work. On a new SSD these numbers should be very small."
This is absolutely wrong. Modern SSDs reserve a significant part of their NAND even when new. For example, the Micron C400 used in the Surface Pro has 128GiB of NAND and reserves 6.8% as spare area, and thus presents to the OS as a 119.2GiB block device. An Intel SSD 525 with the same 128GiB of NAND reserves 12.6% spare area, so it presents 111.8GiB to the OS. The author's MacBook Air seems to have about 11.7% spare area. They're still fundamentally the same amount of storage, but drives with smaller spare area will generally perform worse when they are nearly full, and having less spare area can also reduce the longevity of the drive.
That is true, but in my experience third-party SSDs don't include the reserve space in the advertised drive size.
For example, I have several "60GB" drives that actually have 64GB of internal storage but the extra 4GB is reserved. I also have several "128GB" drive that actually do show up as 128GB to the OS, meaning the reserve space is in addition to the 128GB, not part of it.
Another 120GB drive I has reports the exact same binary size (112GiB) as the Macbook Air "128GB". So it appears that Apple is including the reserve space in the advertised drive size while third-party SSDs do not.
All SSDs advertised as either 128GB or 120GB come with exactly 128GiB of total NAND, with 5-15% reserved as spare area. If the spare area is chosen to be 6.87%, then that exactly accounts for the difference between a binary gigabyte (GiB) and decimal gigabyte, so a 128GiB SSD can have exactly 128 decimal GB of usable space.
Solid state storage is always delivered in capacities that correspond to binary gigabytes, because that's how the chips themselves are sized, packaged, and addressed by the SSD controller. The Surface has 128GiB of NAND (not HDD), and the reserved "spare area" that is not visible to the OS makes the usable area 119.2GiB, or 128 decimal GB.
> "The parts about wear-leveling blacks and bad blocks are just part of how SSDs work. On a new SSD these numbers should be very small."
This is absolutely wrong. Modern SSDs reserve a significant part of their NAND even when new. For example, the Micron C400 used in the Surface Pro has 128GiB of NAND and reserves 6.8% as spare area, and thus presents to the OS as a 119.2GiB block device. An Intel SSD 525 with the same 128GiB of NAND reserves 12.6% spare area, so it presents 111.8GiB to the OS. The author's MacBook Air seems to have about 11.7% spare area. They're still fundamentally the same amount of storage, but drives with smaller spare area will generally perform worse when they are nearly full, and having less spare area can also reduce the longevity of the drive.