Old iMac Running High Sierra

BettyRat wrote:

Sorry I should have been clearer. I entered Recovery mode, disk utility, chose the start up disk and clicked erase. The instructions to 'erase and put back into factory settings' then require you to select 'reinstall macOS', however I am unable to do this step as can't access High Sierra to reinstall.

Hence if I leave it as it is, has my data been completely erased or does the reinstall of the macOS complete the process?..

"Completely erased" is a matter of interpretation. When you erase a hard drive, the computer normally does not write over every section of the drive until all of the data is obliterated. That would take a very long time. Instead, the computer simply marks the space as available for reuse.

This destroys some data in the process, but someone with recovery software who was determined to try to get your data could try to recover data from the disk. They might even get some back. It's likely that any such data would be a jumbled mess, so unless it was worth a LOT to someone, they probably wouldn't go to the trouble of doing it.

With respect to reinstalling macOS, if you've erased the internal drive, reinstalling macOS wouldn't be necessary for your purposes. Providing optical restore discs to the next owner (early Macs) and/or reinstalling a fresh copy of the operating system is about leaving the system in a useful state for the next owner. If the Mac's next stop is the recycling facility, there's no need to put an OS back onto it for that.

Just need to trash machine, but worried that someone could access my data.

I would suggest recycling the machine. I believe that Apple will recycle old Macs for free, and I believe that Best Buy may, also.

It is not quite clear to me what you mean by deleted. Hard disk drive (magnetic media) can be erased, but those erasure methods vary in their effectiveness. Using an industrial bulk magnetic eraser is one method. Erasing using one time or repetitive several pass methods using random data is another but there are means by which data recovery remains technically possible, though expensive and time-consuming.

The method typically used to ensure data remain inaccessible and useless is to use full disk encryption, which Apple calls FileVault. If you used FileVault to encrypt that disk then no such erasure methods are required. You can erase the disk and simply discard it. The hard disk is utterly useless without that password, which is discarded when it is erased.

FileVault complies with NIST media sanitation and secure disposal requirements.

https://nvlpubs.nist.gov/nistpubs/specialpublications/nist.sp.800-88r1.pdf

Intro to FileVault - Apple Support

Other acceptable methods of data destruction include "incineration, shredding, disintegrating, degaussing, and

pulverizing."

So a comprehensive answer requires knowing if that's what you did, or not. In any event "deleting" a hard drive is vague at best.

BettyRat wrote:

Sorry I should have been clearer. I entered Recovery mode, disk utility, chose the start up disk and clicked erase. The instructions to 'erase and put back into factory settings' then require you to select 'reinstall macOS', however I am unable to do this step as can't access High Sierra to reinstall.

If you get an error about the recovery server not being available, then try Fix#3 here:

https://mrmacintosh.com/how-to-fix-the-recovery-server-could-not-be-contacted-error-high-sierra-recovery-is-still-online-but-broken/

Hence if I leave it as it is, has my data been completely erased or does the reinstall of the macOS complete the process?..

It depends on the type of drive involved.

If it is an SSD, then you are good to go.

If it is a Hard Drive encrypted with Filevault, then you are good to go.

If it is an unencrypted Hard Drive (no Filevault), then your data can still be accessed using regular data recovery software.

If the iMac uses a Fusion Drive, then the data on the Hard Drive portion of the Fusion Drive may still be recoverable since it would be an unencrypted Hard Drive (no Filevault).

Just need to trash machine, but worried that someone could access my data.

Is there anything else I can do to ensure all the data has erased? I'm not very technical!

If this Mac uses a Hard Drive (even a Fusion Drive), then you can either use Disk Utility & select the "secure erase" option (where this option is located depends on the version of macOS installer).

Another option would be to create & use a bootable ShredOS USB stick which will write zeroes to the designated drive.....only the Hard Drive needs to be secure erased by writing zeroes to the whole drive. This is not needed for an SSD because SSDs work differently than a Hard Drive.

https://github.com/PartialVolume/shredos.x86_64

You can use the 64bit USB Vanilla .img file as a source for Etcher (Mac, Windows, Linux) to create a bootable ShredOS USB stick.

Well put.