iMac desktop

Removal options depends on the sub-model of the iMac, which we do not know.

iMacs have been in continuous production since 1998 in five broad families and hundreds of variants. There is no single instriuction set to safely access and remove the drive.

Early CRT models can expose you to potentially lethal high voltage if care is not taken. Best left to a pro.

Most were not meant to be taken apart. Those made between 2012 and 2019 have cases that are held together with adhesive and are hard to open. In some models accessing the HD required almost a complete gutting of the computer.

Find your sub-model (the bit with a year) and then web search for that model's takeapart instructions. We are not allowed to post such links here.

If you have no hopes for the device ever again working, you can be more...enthusiastic withthe take-aprt process but you MUST remember that CRT models have an electric viper lurking inside.

Do you want the physical drive out of the iMac to keep it, or do you just want to ensure that your data is secure? If you can still boot the machine from another external drive, here are some ideas of how to securely erase your data.

https://tidbits.com/2022/08/15/how-to-securely-erase-a-macs-ssd-or-hard-drive/

You also might be able to access the internal drive via another computer by booting up in target disk mode, then use another computer to perform the erase of that hard drive.

Transfer files between two Mac computers using target disk mode - Apple Support

"If you have two Mac computers with USB, USB-C, or Thunderbolt ports, you can connect them so that one of them appears as an external hard disk on the other. This is called target disk mode."

OtherWorldComputing has several DIY installation videos for hard drives for different Mac models, you might look one up for your particular Mac.

You will have to disassemble it and detach the screen to get access to the hard drive. Then just unplug it, and unscrew it form its mounting point behind the screen.

You can simply take it to your local Apple retail store and ask them to recycle it for you. 😉