Resetting a Mac is not difficult. Resetting a Mac in the wrong order is expensive, and the expense is never recoverable. So before anything is erased: get the files off, and get the machine out of your Apple account. Those two things, in that order, and then the wipe becomes the trivial part it ought to be.
Getting the files off means a copy you have actually looked at. A Time Machine disk you have never once restored from is a belief rather than a backup. Plug it in, open it, and check with your own eyes that the folders you care about are on it. Then sign out: Apple ID, iCloud, iMessage, and turn off Find My, which is the one people forget and which leaves the next owner with a machine locked to your account. If it is going to be sold or handed on, unpair the Bluetooth peripherals too, purely so the new owner is not baffled.
Apple Silicon machines, and Intel ones carrying the T2 chip, get the easy route. Open System Settings, go to General, choose Transfer or Reset, and the item you want is Erase All Content and Settings. It is the same mechanism iPhones have used for years. It signs you out of Apple services, removes your data and settings, and returns the machine to the setup assistant without you needing to reinstall macOS by hand.
What it actually does under the bonnet is worth understanding, because it explains why there is no going back. The storage in these machines is encrypted at all times by hardware, and the erase does not laboriously overwrite your files — it destroys the encryption keys. What remains is genuinely, mathematically noise. No laboratory can undo it, ours included, and anyone claiming otherwise is selling something.
Without a T2 chip there is no single switch, so it is done the old way. Restart holding Command and R to reach macOS Recovery. Open Disk Utility, turn on View then Show All Devices so you are looking at the physical disk rather than just the volume, select the top-level internal drive, and Erase — APFS for anything reasonably modern, Mac OS Extended (Journaled) for genuinely old machines. Then quit Disk Utility and choose Reinstall macOS.
Two practical notes. The reinstall wants a network connection and will take a while, so do it somewhere with decent wi-fi and the charger plugged in. And on an older Intel Mac that was not using FileVault, an erase is not a shredding: the data is unlinked rather than destroyed, and it can be recovered. If the machine is being sold and it held anything sensitive, either turn FileVault on before erasing or accept that the disk should be physically destroyed instead.
Be clear about what you are choosing. On Apple Silicon and T2 machines, an erase is final and complete, which is exactly what you want when selling and exactly what you do not want when you realise afterwards that the photographs were never actually backed up. On older Intel Macs without FileVault, an erase is superficial; a competent recovery will often bring plenty back, which is fine if you have made a mistake and unacceptable if you have just sold the machine on an auction site.
One thing this site will not do is take on storage soldered to a logic board, and modern Macs are exactly that. If the machine still runs, get your files off it yourself before you erase anything — that is the whole window you have. Once the keys are gone there is no bench in the world that helps, and we would rather tell you that now than take your money to confirm it later.
Sometimes the reason for the reset is that something is already wrong: the machine will not boot, the disk is not being detected, a folder with a question mark appears at startup. That is not a reset problem, and erasing is precisely the wrong response — it removes the only copy of your data in an attempt to fix a fault that erasing does not fix.
In that case, stop. If the Mac has a drive that unclips or unscrews — most iMacs, and MacBooks up to a certain vintage — that drive can be read directly on the bench, entirely independently of whether the machine ever starts again. Post it tracked and insured, or bring it to the Cambridge desk; there is no collection service, so it is one parcel and one phone call. The diagnostic is free, finished inside 2 working days of arrival, and produces a fixed written figure before anyone commits you to anything.
Files out first, wipe afterwards — there is no later stage at which the order stops mattering. Both erase routes are final, and once the keys in a modern Mac's soldered, encrypted storage have gone, no laboratory anywhere is retrieving what was on it.
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