How to "seriously" securely erase a Late 2015 iMac I7 for resale

"Secure erase" has been gone in macOS for quite a while, if for no other reason that it could never accomplish what that phrase implies. Flash storage used in all recent Macs finally rendered the old multi-pass attempts completely inappropriate and obsolete.

NIST recommends full disk encryption, which for Macs means FileVault. As long as you used FileVault, all you need to do is erase it. Its data are utterly irretrievable without the FV encryption key — which itself gets erased upon erasing the disk.

Follow the steps at > What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

1) when preparing an older Mac, skip over the "Use Erase All Content and Settings" section.

2) APFS is the correct format for reinstalling High Sierra or a later macOS.

As the other have said, there is no reason to secure erase.

There is no "Erase All Content and Settings" offered for that year model iMac.

It also sounds like your Fusion Drive is now split into two separate drives.

To fix the split Fusion Drive, see > How to fix a split Fusion Drive - Apple Support

It sounds like you figured it out.

A simple erase of the physical SSD is sufficient to destroy the data on it due to how SSDs work (especially with TRIM enabled -- which it is for an internal Apple OEM SSD).

For the internal Hard Drive, you will need to use the "Secure Erase" option within Disk Utility. That option only appears when the target is an actual Hard Drive.

If you want to reinstall macOS for the next owner, then will first want to setup the Fusion Drive using the instructions in the following Apple article:

How to fix a split Fusion Drive - Apple Support

Then you can quit the Terminal app and select Disk Utility to confirm the creation of the Fusion Drive used the correct file system for the OS you are going to install. It should be using the APFS file system for macOS 10.15 Catalina and later. Quit Disk Utility and select "Reinstall macOS" option.

I think secure erase is for HDD's, not SSD's. The magnetic info is harder to erase.

Unless you keep national secrets on your computer what Den.Thed has provided is completely fine. If you keep national secrets on it remove the drive and then shred it through an industrial grade shredder.

Robert Garven wrote:

I know these are not worth much but it has a newer 3 TB drive and hoping someone may want it

That would make this iMac very attractive.

While macOS 12.x Monterey is still fairly recent, there are many third party apps that will no longer be supported by their developers. You can still run the older versions of these third party apps, but sooner or later they will start to have issues if they require accessing any online services since those apps may no longer be receiving any new updates. Nobody knows when this may happen, and it may happen gradually to some apps before other apps & services.

Once macOS is no longer useful, I would personally consider installing & using Linux Mint on this iMac although Linux does not support a Fusion Drive, but Linux may be able to fit completely on the small internal SSD or it could be customized to install the core OS & apps on the small SSD and have your home user folder with all the data on the slower internal Hard Drive.

Linux is a non-Apple OS, but is a great way to extend the useful life of older computers. Linux does require learning a new OS, but Linux is not for everyone. The Linux Mint software repositories have a lot of free open source apps such as LibreOffice (an office suite like Microsoft Office & is installed by default), GIMP (something like Photoshop), Firefox web browser, etc. Plus many of the popular third party web browsers are available to be downloaded & installed (Vivaldi, Google Chrome, Brave). Even Black Magic's Davinci Resolve video editing software is available for download & installation on Linux (from what I understand from a YouTuber I follow, it works really well on Linux). Unfortunately many popular third party proprietary software such as Adobe products, Microsoft Office, etc. are not available for Linux although you can use the web browser to access the Office 365 online version. Video streaming services are not well supported (no streaming service apps like Netflix, Disney+, etc., and the web browser access is limited to 720p at best due to the paranoia of the media/streaming industry). There are a fair number of popular games that can run on Linux, but there are lot that won't work (especially for the online only games which utilize anti-cheat....one type may work, but the other won't). Most of these popular games are going to be Windows versions of the games running through a compatibility layer called Proton (something similar is used on macOS as well).

You need to install the older offered macOS version as per the "How to reinstall macOS" article below and then let the new owner upgrade the macOS using their own Apple ID.

see > How to reinstall macOS - Apple Support

• If you used Option-Command-R to start up from Internet Recovery, you might get the latest macOS that is compatible with your Mac. 
• If you used Shift-Option-Command-R to start up from Internet Recovery, you might get the macOS that came with your Mac, or the closest version still available. 

Robert Garven wrote:

Unfortunately, I somehow split up the fusion drive and the solution in the link above does not work. I erased my operating system so I'm starting it up in Internet recovery mode and and I'm trying to install a new Monterey OS. It wouldn't let me put it on the smaller SSD drive, (operation failed) so now I'm trying to install it on the 3 TB media drive, and that just failed. Any suggestions?

The best thing would be to recreate the Fusion Drive using the instructions in the Apple article I linked previously. You will need to first use Disk Utility to unmount both the internal SSD and the internal Hard Drive before you can use the commands in the Terminal. I'm not sure the Apple article mentions this part.

so I wiped both drive and hope I can get it back to factory settings.

I tried erasing and reformatting the drives neither would take APFS, only journaled/guild. Now I can't restore the OS both failed.

The APFS option will only be available when booting into macOS 10.13+ (High Sierra and later). Plus you must erase the whole physical drive as opposed to just a volume/partition on the drive. The partition type must also be GUID.

If you recreate the Fusion Drive, then only the "Fusion Drive" item is what you need to erase. However, I think the Fusion Drive creation process will automatically format the Fusion Drive properly for the OS installer you have booted & will install.

If you are booting macOS 10.10 to 10.12, then you can only select MacOS Extended (Journaled) file system.

With macOS 10.13 to 10.15 the file system can be either MacOS Extended (Journaled) or APFS (top option) since the installers will convert the file system automatically to APFS.

With macOS 11.x+, the file system must be APFS (top option) since those installers will no longer automatically convert the file system, although they may allow the first copy phase to complete, then fail when rebooting to phase 2 which performs the actual installation.

With macOS 10.13+, within Disk Utility you may need to click "View" and select "Show All Devices" before the physical drives and "Fusion Drive" items to appear on the left pane of Disk Utility.

There are two common ways of accessing Internet Recovery Mode (assuming internal drives erased):

• Command + R -- Should boot into the online installer for the last OS installed before it was erased
• Command + Option + R -- Should boot into the most recent online installer compatible with the Mac (for a Late-2015 iMac that should be macOS 12.x Monterey)

However, some Macs may only boot into the online installer for the version of macOS which originally shipped on the Mac from the factory (which should be macOS 10.11 El Capitan) regardless of the keys used for booting.

Unless your computer has PII, or HIPAA compliant data, the standard erase should be fine.

Here is a link to the new thread:

Recovery Mode will not reinstall Monterey… - Apple Community