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tldr: how do I erase all of the information from my startup disk? Logging into recovery mode and selecting "erase" has no affect on my disk's "free" space.

While attempting to diagnose an issue I was experiencing, I decided to erase my iMac's startup disk and then restore from a time machine backup. So I log into recovery mode, open up disk utility, and select "erase". Somewhat surprisingly, I see that, while the disk's "used" space drops to almost zero, all of the data seems to be transferred to the "other volumes" category, and the amount of "free" space on the disk doesn't change.

I assumed that this was simply a labeling issue, and that the operating system had earmarked all of this "other volumes" information to be deleted asap. So I opened up time machine and restored from backup. After having completed the time machine restore, I logged into find that all of that "other volumes" data was still taking up hundreds of gigabytes of space, and I had only ~20GB free on the computer.

So I try logging into recovery mode again and selecting "erase." This time it does free up all the "used" space, but it still leaves ~500GB of "other volumes" data.

So how do I actually erase all of the information on my startup disk? Logging into recovery mode and selecting "erase" seems to have no affect. Thanks!!

  • Can you edit with the output of diskutil list in Terminal to your question? – grg Aug 11 at 17:41
  • @grg unfortunately I've already figured out how to reformat the drive which fixed the issue. – John Aug 12 at 3:34
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This will clear all the data on your drive!

You can erase the whole volume by clicking the View button and choosing Show All Devices. Then select the top-level entry and click Erase. Set it to APFS and GUID and click Erase. Name it Macintosh HD (or whatever you prefer if that's not your preference). Then run the installer on your new Macintosh HD and you'll get a clean install with no data.

  • are you doing this from Disk Utility? – jmh Aug 11 at 17:24
  • This isn't going to work if you're booted from that drive. – Tetsujin Aug 11 at 17:50
  • @Tetsujin Yes it will, when you're in recovery mode you can erase the whole disk, even though it includes the recovery partition. I've done it before. If you restart the machine there's no recovery mode left, but the way it works doesn't require it to continue existing once it's running. Evidence for this (beyond trying it) can be found by observing the mounted disk image in Disk Utility when in recovery mode. I believe this is where the working data is kept. – Ezekiel Elin Aug 11 at 18:31
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Ah HA! Well it took about 24 hours to figure out, but the solution is to:

  1. Boot into recovery mode
  2. Open disk utility
  3. Select the internal drive in the side bar
  4. Find the "+/-" volume button in the toolbar and select the "-" volume option.
  5. The appearance of the internal drive will change in the sidebar. Select the root (top level) of the drive in the sidebar and reformat the drive (I'm afraid I don't quite remember which button allows you to reformat the drive, but I remember it being somewhat obvious).
  6. This time the options presented to you allow you to choose the file system as well as the partition scheme, when you reformat the drive everything is successfully erased.
  • It's usually easier from Internet Recovery, which sets up the recovery partition in RAM - otherwise it's a bit like sawing off the tree branch you're standing on, which the system should prevent you from doing ;) – Tetsujin Aug 11 at 16:16

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