Steps to clean an iMac hard drive before disposal

If the old Mac is still functional use Disk Utility to perform a secure erase. The easiest way, by far, is to take the drive out and use a 5 lb hammer and smash it to bits.

Or use Apple’s official process.

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

• If you encrypted the startup disk with FileVault no special procedure is required. Just erase it.
• If you did not use FileVault, then physical destruction; specifically:

Disintegrate

Incinerate

Melt

Pulverize

Shred

Refer to the NIST Guidelines for Media Sanitization.

Erasing a FileVault encrypted storage device is designated "cryptographic erase" as that document describes.

As far as the "secure erase" multipass procedures previously incorporated in Disk Utility and similar utilities, it states:

"... such practices should be avoided as very little confidentiality protection is achieved."

Our friend, @Ikrupp, has provided the best guidance.

I'll simply add that if your intention is to send the iMac to be recycled, then steps 5 & 6 in the document they linked are unnecessary. There's no need to reinstall the OS and reset the NVRAM.

Smashing the drive with a hammer is certainly a secure method, but unless you want to end up with a large pile of broken glass and plastic from taking the hammer to the entire machine you may find it a lot of work to get the drive out of any but a truly ancient iMac.

John Galt wrote:

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As far as the "secure erase" multipass procedures previously incorporated in Disk Utility and similar utilities, it states:

"... such practices should be avoided as very little confidentiality protection is achieved."

FWIW, multi-pass overwrite hasn’t been necessary since the era of floppy disks and 1980s-era hard disks; on gear not using so-called embedded servo tracking for head positioning. The older gear had sloppy positioning. Newer storage gear is immensely more dense, and tracking is immensely more accurate. And multi-pass overwriting is completely unnecessary on SSDs.