Every file and folder on your computer has an invisible set of rules attached to it: who is allowed to open it, who can change it, and who can run it if it's a program. Most of the time you never see these rules because you're the only account on the machine and you own everything. They become visible — usually as an error message — the moment you share a computer, plug in a drive formatted somewhere else, or try to access a file that belongs to a different user account.
The Three Basic Permissions
Both Windows and Mac trace back to the same three ideas, borrowed originally from Unix:
- Read — open and view the file, or list a folder's contents.
- Write — modify, rename, or delete the file, or add/remove items in a folder.
- Execute — run the file as a program, or, for a folder, enter it and access what's inside.
These three permissions can be granted separately to the file's owner, a group of users, and everyone else — which is why you can own a spreadsheet, let your team edit it, and let the rest of the company only view it, all with the same file.
How Windows Handles It: NTFS Permissions
Windows uses a more granular system than the classic three permissions, called NTFS permissions, visible by right-clicking a file or folder → Properties → Security tab. Instead of three levels, Windows offers six: Full Control, Modify, Read & Execute, List Folder Contents, Read, and Write, each assignable to specific user accounts or groups rather than just "owner/group/everyone."
- Right-click the file or folder, choose Properties, click the Security tab.
- The "Group or user names" list shows every account or group with permissions set. Click one to see its specific rights below.
- Click Edit to change permissions — you need to be the owner or an administrator to do this for most files.
- The Advanced button exposes inheritance settings — whether a folder passes its permissions down to everything inside it, which is the default and usually what you want.
A common source of confusion: a file can show "Access Denied" even to an administrator account if its owner is a different user and inheritance was broken at some point — often after copying files from another PC or restoring from a backup made under a different account.
How Mac Handles It: POSIX Plus ACLs
macOS is built on Unix, so it uses the classic owner/group/everyone model directly, visible via Get Info (Cmd+I) on any file, under "Sharing & Permissions" at the bottom of the panel. Each entry gets Read & Write, Read Only, Write Only (Drop Box), or No Access.
- Select a file or folder, press Cmd+I.
- Scroll to "Sharing & Permissions" and click the lock icon to unlock editing (requires an administrator password).
- Click the + button to add another user or group, or change the permission level next to an existing entry using the dropdown.
- The gear icon offers "Apply to enclosed items" to push the same permissions down into every file inside a folder.
macOS also supports Access Control Lists (ACLs) for finer-grained rules layered on top of the basic three, though most people never need to touch them outside of shared file server setups.
Why a File From One System Won't Open on Another
Plug in an external drive formatted for one system and used under a different account, and permissions can block access entirely even though the file is technically right there. This is especially common with drives moved between a work Mac and a personal Windows PC, or after cloning a drive rather than copying files individually — the clone carries over the original owner information, which the new machine doesn't recognize as belonging to any of its own accounts.
The Terminal Shortcut (When the GUI Won't Cooperate)
On Mac, chmod and chown in Terminal fix permission problems the Finder's Get Info panel sometimes can't touch — for example, sudo chown -R yourusername ~/Documents/ProjectFolder reclaims ownership of an entire folder tree recursively. On Windows, the equivalent is icacls from Command Prompt or PowerShell, which can reset permissions on a folder tree with icacls "C:\Path" /reset /T. Both commands are powerful enough to lock yourself out if used carelessly — read what a command does before running it as administrator or with sudo.
Practical Cases Where This Matters
- Shared family computers: keep a private folder that other accounts can't open, rather than relying on people simply not looking.
- External drives used across systems: formatting as exFAT sidesteps most permission conflicts since exFAT doesn't carry ownership metadata the way NTFS and Mac's APFS do — see also our guide on encrypting a USB drive, which touches on cross-platform drive formatting.
- Downloaded files that won't run: on Mac, files downloaded from outside the App Store sometimes need Execute permission granted manually or an explicit override in Privacy & Security settings before they'll launch.
Permissions exist to prevent accidents, not just malicious access — most people run into them the first time by locking themselves out of their own file, usually after a permissions change made while troubleshooting something unrelated. Understanding the three-permission model underneath both systems makes those error messages a lot less mysterious. For a deeper look at how file types and formats interact with permission systems, see our guide to file formats.