The reset itself takes a few minutes and almost nobody gets it wrong. What people get wrong is the running order, and that mistake cannot be undone afterwards. Two jobs come before the erase: move the files somewhere else, and take the machine off your Apple account. Handle those and the wipe is the boring five minutes it ought to be.
Moving the files means a copy somebody has actually opened. A Time Machine disk nobody has ever restored from is an assumption rather than a backup, so mount it and look for the folders that matter to you by name. Then deal with the account side: sign out of Apple ID, iCloud and iMessage, and turn Find My off. That last switch is the one people skip, and skipping it hands the next owner a machine tied to your account. Unpair the Bluetooth keyboard and mouse as well if the Mac is going on to somebody else.
Anything with Apple Silicon, and any Intel Mac carrying the T2 chip, has a single switch for this. System Settings, then General, then Transfer or Reset, then Erase All Content and Settings. iPhones have worked this way for years, and the Mac has borrowed the idea wholesale. One click signs you out of Apple services, clears the data and settings, and drops the machine back at the setup assistant with no manual reinstall of macOS involved.
It is worth knowing what happens underneath, because it explains why nothing comes back afterwards. The storage in these machines is encrypted by hardware from the day it leaves the factory. The erase does not grind through your files overwriting them. It throws the keys away, and what is left behind is noise in the mathematical sense. Nobody reverses that, this bench included, and any firm telling you otherwise is selling something.
No T2 chip means no single switch, so the job goes the long way round. Hold Command and R while the machine restarts, which brings up macOS Recovery. In Disk Utility, tick Show All Devices under the View menu first, because you want the physical disk and not one of the volumes sitting on it. Select the internal drive at the top of the tree and erase it, choosing APFS on anything reasonably recent and Mac OS Extended (Journaled) only on genuinely old hardware. Quit Disk Utility afterwards and pick Reinstall macOS.
Two things worth knowing before you start. The reinstall pulls macOS down over the network and is not quick, so sit the machine somewhere with reliable wi-fi and leave the charger in. The second matters more. On an older Intel Mac with FileVault switched off, erasing is not shredding. It unlinks the data and leaves it lying where it was, and a competent recovery will pull plenty of it back. That is a relief if you have wiped the wrong machine and a serious problem if you have just posted it to a stranger from an auction site. Turn FileVault on before you erase, or take the disk out and have it destroyed.
There is a point where erasing stops being enough. A Liverpool office clearing a cupboard of retired drives, a solicitor disposing of closed case files, a clinic with patient records: none of them want a disk leaving the building readable, and a quick format leaves it entirely readable. Two things genuinely destroy data on a spinning disk. Overwriting every sector, which takes hours per drive and needs the drive healthy enough to accept the writes. Or destroying the platters physically. A hole drilled through the casing is not that, because a drill misses most of the surface and the surviving fragments can be read on a bench by anybody patient enough to try.
Flash is different again. Overwriting is unreliable on an SSD or a memory card because the controller moves data around behind your back, so the sound approaches are a hardware secure erase, a cryptographic erase where the drive was encrypted from the start, or shredding to a small particle size where the data was genuinely sensitive. This bench is not a destruction contractor and will not pretend to be one: for a quantity of retired drives, use a firm that shreds to a documented standard and issues a certificate you can put in front of an auditor. What we can tell you is what happens to media we hold. It is returned to you or securely destroyed on your written instruction once the job is finished, and nothing goes in a skip.
Quite often the reset is being attempted because something has already broken: the Mac will not boot, the disk has stopped being detected, or a flashing folder with a question mark turns up instead of the desktop. None of that is a reset problem. Erasing is the worst available answer to it, because it throws away the only copy of your data while doing nothing whatever about the fault underneath.
Stop there instead. Where the Mac has a disk that unclips or unscrews, which covers most iMacs and MacBooks up to a certain age, that disk reads perfectly well on a bench whether or not the computer ever starts again. Send it tracked and insured to the lab at Peter House, Oxford Street, Manchester, roughly 35 miles down the M62. No collection service exists, so it comes down to one parcel and one call on 0800 689 0668. Diagnosis costs nothing and is done within 2 working days of the parcel landing, and any single drive or SSD is £300 + VAT, put in writing before a penny becomes chargeable. Storage soldered onto a logic board is the one honest no on this page, and it is better said now than after you have paid somebody to confirm it.
Files out first, wipe second. No later step makes up for getting that order wrong. Both routes erase for good, and on a modern Mac the encrypted storage is soldered down and keyed. Once those keys go, what was on the machine is beyond every laboratory on earth, this one included.
Free diagnostic within 2 working days of arrival, one fixed written quote, no fix no fee on logical faults. Cards £250 + VAT, any one drive or SSD £300 + VAT.