How do I completely wipe the hard drive on an old iMac?

unless you wish to physically damage the drive then this should be enough

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

unless you're a secret angent then it will likely be enough as, the time effort and money it would cost people to attempt to salvage data from the hd would be high and unless your data was real "gold" then likely not worth it.

You need to select Disk Utility and Erase the Hard Drive.

Again, the instruction are at > Use Disk Utility to erase an Intel-based Mac - Apple Support

You can not erase the drive that you are started up from.

You must startup in macOS recovery mode as per the article that Rudegar posted,

or from the original gray install disc if it is a 2010 or earlier model iMac.

Use Disk Utility to erase an Intel-based Mac - Apple Support

Vince,

NEVER EVER post private information (names, addresses, phone, e-mail, etc.) on a public forum. I have requested that the moderator remove yours.

If you use Apple Recycling Programs this service is free and they will "heroize" the HD too.

You were told already, please look at my last post where it clearly said:

• Go into the Recovery Partition, open Disk Utility
• Erase the internal HD.

You can't erase the volume that you are currently booted from. It is like asking a mechanic to disassemble the engine of a car that is still running. Not going to happen.

You need to boot into macOS Recovery (as the Apple Support articles describe), or boot from an external drive containing an installation of macOS, to be able to erase the internal installation of macOS.

On very old Macs that predate Recovery mode and that came with optical discs for reinstalling Mac OS X, you would boot the Mac from the recovery CD/DVD and proceed to erase it and reinstall the OS from there.

vbbuilt wrote:

Well, I explained earlier the erase option under Disk utility is gray out, no joy, thanks.

Vince

You must be booted from a different volume than the one that you are trying to erase.

You need to use Disk Utility while booted into Recovery – in which case, you would probably be allowed to erase the partition containing the main macOS installation, but not the Recovery partition. Or use it while booted from an external drive.

If you have a bootable clone backup around, you could boot off of it and then erase the internal drive.

If you don't, you could put a copy of macOS onto an external drive (either via clean install, or by making a clone backup with Carbon Copy Cloner or SuperDuper!), boot from that external drive, then erase the internal drive.

With the keyboard plugged into the back of the computer.

Shut down the computer.

Turn on and hold down Command + R until it starts.

Go into the Recovery Partition, open Disk Utility

Erase the internal HD.

You need to use Disk Utility to Erase first and then Re-Install the macOS.

You need to boot into the Recovery Volume and open Disk Utility there and format the drive. If you cannot do that, when you send it in for recycling they will erase the drive for you.