I am just wondering when using the option erase all content and data how this is done from a system side, does it format a partition on the memory or does it just delete it remove indexes like a normal delete? I would imagine the system files are pretty much untouched? Example scenario I am imagining here is;

A) system partition 2GB ish
B) recovery partition, no idea
C) user space, everything else

So when a factory reset happens does C have it's data deleted or just totally formatted? If the later would it skip empty cells on a format?

For example Joe Bloggs gets a new iPhone, usage is 20gb/120gb. 3 days later factory resets it, does it use one of the flash memories finite writes on the entire partition using limited writes or skip over it and write over just the 20gb? I am assuming here that the memory in each iPhone cell can take around 20,000 wipes anyway, so would each cell go down to around 19,999 on average or just the cells that used to have data on before the format if there is one?

2 Answers 2


Actually it is both more complicated as well as simpler!

When you choose to erase all contents from the device, the only thing overwritten and deleted is the encryption key for the device. Without the key, the contents of the disk drive is no longer accessible to anyone, and is thus treated as being equivalent with it being deleted.

In your example, neither 120 GB nor 20 GB of data is written to the disk. It is only a very small amount (less than 1 MB) that is written. Namely the encryption key(s) are overwritten, and the drive is formatted a new (which only writes certain data structures to the drive - it does not as such overwrite the old data contents of the drive, neither used or non-used).


When flash NAND get's wiped, it's not actually erased. It's marked as unused and depending on the technology used, it will either return garbage or zeros. When that space is asked to store something, it's at that point it gets overwritten.

Since iOS (as of 10.3) shares the APFS filesystem that macOS uses, we can reasonably say that file maintenance is essentially the same. How deletions are handled depends on how it's implemented in the hardware.

Can the data on an MacBook Pro SSD be recovered after formatting using Disk Utility

Secondly, there isn't a hard limit on the number of writes - yes, there is a finite limit, but it's a calculated value. It's not going to be like there's exactly 20K writes and the moment you go from 19,999 to the next one, it stops working. This is similar to charge cycles on a battery - it doesn't stop working at 1000 cycles as many people seem to believe. Additionally, there isn't a "limiter" that will turn off your drive when it reaches a certain point. There are however, technologies built into the hardware like wear leveling that ensures consistency across the entire SSD.

So when a factory reset happens does C [user space, everything else] have it's data deleted or just totally formatted? If the later would it skip empty cells on a format?

On a full factory reset? Yes. However, you can reset the iPhone without removing all the data and settings. When it's formatted, it will mark the used cells as unused and any attempt to read them will return gibberish or zeros. Unused cells would be skipped.

  • I was always under the impression that a full reset on an iPhone simply removed the encryption keys, then set up a new filesystem to install to; then encrypted afresh with new keys.
    – Tetsujin
    Aug 25, 2020 at 8:38
  • No, you cannot reasonably say that file maintenance is necessarily the same on macOS as on iOS. In fact it differs quite a lot. It is only Macs with the M1 chip share the same manner of operation as iOS devices (and those Macs didn't exist when this answer was written). The big difference is that files in iOS are run through what is known as "Data Protection" on iOS. That is not the case on macOS on non-M1 Macs. Indeed even if you have an M1 Mac, it is still not the same, as the protection is more less granular on the M1 as the protection is per volume, and not per file or per extent as on iOS
    – jksoegaard
    May 22, 2021 at 8:52

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