Wiping a Mac is easy. Wiping a Mac in the wrong order is how people lose fifteen years of photographs in ninety seconds. The order is: get the files off, check them, sign out of everything, then erase. Not one of those steps is optional and the last one is irreversible, which is the whole reason the first three come before it.
Get the files off properly. Copy to an external drive, or to iCloud, or use Migration Assistant onto the new machine, then open some of them. A folder that copied without an error message is not the same thing as a folder whose files open. Check the photo library actually renders, the documents open, and the desktop and Downloads folders came too, because those two get forgotten more than everything else combined. Then sign out of iCloud, turn off Find My, sign out of iMessage, and — this is the one people miss — deauthorise the machine in Apple Music if you ever used iTunes on it. Finally, unpair anything Bluetooth so the new owner does not inherit your keyboard.
Any Mac carrying Apple Silicon or a T2 chip — effectively everything sold from 2018 onward — has one correct route, and Disk Utility is not it. Go to System Settings, then General, then Transfer or Reset. The item you want is labelled Erase All Content and Settings, and it behaves just as its iPhone equivalent does: signs you out, strips your data, and hands back a machine sitting at the setup assistant.
What that actually does is worth understanding, because it explains the whole of the last section. Flash in these machines is encrypted permanently, in hardware, whether or not FileVault was ever switched on. Erase All Content and Settings destroys the encryption keys. The bits on the flash remain, but without the keys they are noise and stay noise. That is why the process takes a minute or two rather than an hour, and it is also why nothing survives it. If the option is greyed out, the machine is either on an older macOS or is not signed in — update it, sign in, and it appears.
On a pre-2018 Intel Mac with no T2, there is no single button and you do it by hand. Shut down. Hold Command and R while powering on to reach macOS Recovery. Open Disk Utility, choose View then Show All Devices so you can see the physical disk rather than only the volume, select the top-level drive, and click Erase. Format as APFS on anything reasonably modern, or Mac OS Extended (Journaled) on genuinely old hardware, with GUID Partition Map as the scheme. Then quit Disk Utility and reinstall macOS from the Recovery menu.
On these machines FileVault genuinely matters, because the storage is not encrypted by default. If FileVault was on, erasing destroys the key and that is that. If it was off, an erase leaves recoverable remnants on a spinning drive — SSDs behave differently, since TRIM clears deleted blocks fairly promptly. Selling an old Intel MacBook with a mechanical drive and no FileVault? Turn FileVault on, let it finish, and then erase. Ten minutes of patience closes the gap entirely.
Thorough. On any modern Mac, once the keys are gone the data is gone, and no laboratory anywhere gets it back — including this one. There is no clever recovery for a correctly erased Apple Silicon or T2 machine, and anybody claiming otherwise is selling you something. It is a good design and it is the right one for a machine that changes hands.
The corollary is the part that catches people out. Because a modern Mac's flash is soldered to the logic board and locked to that board's encryption, there is no removing the storage and reading it elsewhere. Storage soldered to a motherboard is the one thing this bench cannot take, on any make. Which means the copy you made before the wipe is not a precaution, it is the only copy — check it opens before you press the button, not after.
A lot of the Macs that reach this bench were never being wiped on purpose. The machine failed and somebody reached for a reset, or a student at Aberdeen or Robert Gordon reinstalled the system to fix a boot loop with a dissertation still on the disk, or a reset was started and interrupted by a flat battery halfway through. Interrupted erases are the recoverable ones, because the process is often not as complete as the progress bar suggested.
The general rules hold. On an older Mac whose drive or SSD blade unscrews, the storage comes out and is read on its own regardless of what killed the machine, and a dead Mac is frequently a straightforward recovery. On a 2018-or-later machine the flash is soldered down and encrypted to the board, and that is outside what we take — we would rather say so on the phone than after you have paid for postage. If it is the older kind, the diagnostic costs nothing and is finished within 2 working days of arrival, and you get one fixed written figure before anything begins. Ring 0800 689 0668 and one question settles which machine you have.
Files off first, wipe second — the order never stops mattering. Both routes are irreversible, and once the keys inside a modern Mac's soldered, encrypted flash are destroyed, nothing returns the data. Copy it, open what you copied, satisfy yourself it is all there, and only then reach for the button.
Diagnosis costs nothing and takes 2 working days from arrival, the figure is fixed in writing, and logical work runs no fix no fee. Begin online, or lift the phone.