I need to downgrade from Mojave to High Sierra but am not sure if I can safely erase the HDD in order to install the new OS.

I'm aware that I can create an installer on a USB drive for High Sierra or I can reinstall the version of Mac OS that the machine came with using Internet Recovery mode. Regardless, I know that I'll need to erase the HDD. I was about to do so but it gave a warning message that erasing the HDD will also erase partition disk0s2 and I'm not sure what that partition is for.

Inputting diskutil list into terminal outputs the following: enter image description here

After downgrading to HS, I will move over files that I need from the a time machine backup I created on Mojave. I’m aware that I can’t install the whole backup from time machine as the backup was taken on Mojave.

Any help will be much appreciated. Thanks.

  • Not sure why you'd really want to - I found both the Sierras unbearable, yet Mojave, which I'm still on for my main Mac, settled out nicely. Still runs the 32-bit stuff too.
    – Tetsujin
    Jul 31, 2022 at 17:54
  • BTW, such as your Mail database will not go 'downhill' unless you have a copy from before. Also see apple.stackexchange.com/questions/271104/…
    – Tetsujin
    Jul 31, 2022 at 17:55
  • You edit has completely changed the emphasis of your question. We cannot answer without knowing what's on that partition. diskutil list or diskutil cs list may give a clue. Please add info to your post. bear in mind this may depend on what you are currently booted to. Nothing before Sierra knows what an APFS partition is. 'Safe' always depends on how good your backup strategy is.
    – Tetsujin
    Jul 31, 2022 at 18:24
  • Thanks, @Tetsujin. High Sierra on this Mac has always been pretty okay and as soon as Mojave was installed the other day, it has not been a good experience. It's not a new machine. I've updated the question to be clearer and have included a screenshot as advised. Many thanks.
    – MrFish
    Jul 31, 2022 at 19:02
  • If you have a HDD, then installing High Sierra on a JHFS+ partition will be a much faster OS over Mojave on a required APFS partition. In other words, macOS installed on a APFS partition really needs to be done on a SSD. What model/year Mac is your machine? Jul 31, 2022 at 20:45

1 Answer 1


Have a look at the following tutorial regarding how apfs uses containerized disks. It'll clarify your question. Nice pics in the writeup, too. I found it helpful.

Short answer, "erasing disk0s2" is "erasing your old hard dive"


  • Thanks a lot, @buzzlms. So, essentially, erasing my HDD with this partition "disk0s2" that I didn't even know I had will all be fine, right? I'll then be able to install HS with an installer I've created on a USB drive and can then put my files back on from the backup I've made.
    – MrFish
    Aug 2, 2022 at 0:43

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