You bought a second-hand Mac. Here is how to make sure the space is all yours
A used Mac can still carry the last owner's accounts, apps and files. How to check, and why erasing and starting fresh is almost always the right call.
A used Mac should arrive erased, but many do not. The last owner's account, apps and hidden files may still be on it, using space and sometimes their accounts. The clean way to start is to erase it.
Check what is on it
# Accounts on this Mac
ls /Users
# Space used right now
df -h /System/Volumes/DataIf you see accounts other than yours and Shared, or far more space used than a fresh Mac, the previous owner's data is still there. Old user accounts covers what they cost.
Check it is not locked
Before anything else, make sure the Mac is not tied to the previous owner's Apple Account. In System Settings, General, About, or during setup after erasing, an Activation Lock prompt means you need the previous owner to remove it.
Erase and start fresh
On Macs with Apple silicon or a T2 chip on macOS Monterey or later, System Settings, General, Transfer or Reset, Erase All Content and Settings wipes everything and leaves macOS installed. It is the cleanest start.
On older Macs, erase the disk from Recovery with Disk Utility and reinstall macOS.
After setup
A fresh Mac uses a fixed amount for macOS and its system volumes. What uses space on a new Mac shows what to expect, and why a 256 GB Mac shows less explains the difference from the label.
Common questions
Should I erase a used Mac I just bought?
Yes, almost always. It removes the previous owner's accounts, files and apps and gives you all the space.
How do I erase a used Mac?
On Apple silicon or T2 Macs with a recent macOS, use Erase All Content and Settings in System Settings, General, Transfer or Reset. Older Macs are erased from Recovery.
What if the used Mac asks for the previous owner's Apple Account?
That is Activation Lock. The previous owner must remove the Mac from their account before you can use it.