In OS X, file permissions can be a problem once in a while.
Every file on your Mac – even the hundreds of “invisible” files that you don’t normally see that are in the core of the operating system – have what are called permissions. These are attributes that say who “owns” the file and who has the rights to open it and use it. All modern operating systems use an “account” structure to organize user files and user permissions to do things, and one of the ways that works is by assigning permissions to each file for each owner. Different accounts for different people wouldn’t mean much if anyone of those people could open everyone’s files.
In your case, the file you got either had permissions that OS X couldn’t read, or no permissions at all, and OS X didn’t quite know what to do with it. All files have permissions, including a typical document, a program, a photo; any file on your Mac. Folders even have permissions, too.
And because the core of OS X is a version of Unix, a multiuser operating system with a long history behind it, every folder and file has the capability of having permissions sophisticated enough to be able to handle hundreds of users.
These permissions are generally one or more of these basic three: Read, Write and Execute. Those mean one has the permissions to open and read a file, or write to a file (make and save changes), and execute a file, which means run it like a program.
Those permissions can be mixed and matched, too. A file can readable and writable but can’t run like program. or, a file can be read and run like a program, but not changed.
What does this all mean? Usually nothing, because most of the time your file permissions stay where they are set and don’t change. File permissions get a quick check each time you start or restart your Mac. and if something isn’t quite right with the permissions, it gets fixed on startup and you don’t even know it.
When you get a file email or from another disk or flash drive, copying it into your account should set the permissions correctly. But once in a while you’ll get a message saying you don’t have sufficient permissions to do what you want to do. That’s where setting or repairing permissions comes in.
But first: if you look at the information that’s available for a file, you can see what I mean by the permissions for each file. You can also change them individually this way, but that’s not a good idea unless you know what you’re doing.
Click once to highlight a file and then select “Get Info” from the Finder under the File menu. Or select a file and then use Command-I. That’s hold down the Command key – the one with the Apple and or Clover shape, on each side of the space bar – and then hit the “I” key. You’ll get an info box that has lots of stuff in it, and what we’re looking for is farther down on the bottom.
It’s called Ownership and Permissions, and open it by clicking on the little triangle. You’ll see one drop down box, and it should say that you can “Read and Write” the file. If it only says “Read Only” or “No Access” and it’s a file you got from someone or a file you’ve used before, it’s generally OK to change that to Read and Write.
There is another triangle below that with “Details,” and these you shouldn’t play with too much. If a file isn’t owned by you in your account – listed as your account name in the “Owner” attribute – and you know it should belong to you, try repairing the permissions of your hard drive as below.
You’ll notice a little lock next to the Owner; that’s because you’ll need administrator privileges to change that, and if you’re using a non-administrator account, you won’t be able to change the owner.
If you have trouble with a flash drive not letting you copy files in OS X, you can check the permissions of the drive itself. Highlight the drive on the desktop and then “Get Info” on it from the File menu, or hold down the Apple key and hit “I”. You’ll see in the get info window the options for “Ownership and Permissions.” The first one – “You can” – should be “Read & Write.” If it’s not, change it.
(The tick box “Ignore ownership on this volume” has to do with the drive being bootable under OS X. Making bootable backups is a subject of another Q and A. (There’s an interesting bug in 10.4 that has to do with making bootable backups.)
If you find yourself with lots of permission errors, you can verify permissions for the whole hard drive. It’s a good thing to do once in a while anyway. Use Disk Utility in the Utilities folder. Start it up and then select your hard drive in the left hand column. Click on “Verify Permissions” to check things without changing anything, or “Repair” to fix problems.
Apple recommends repairing permissions without starting up from a different disk or installer CD or DVD, according to this article “About Disk Utility’s Repair Disk Permissions feature:”
When possible, disk permissions should be repaired while started up from a Mac OS X volume (hard disk) that contains updated Mac OS X software, instead of a Mac OS X installation disc. Mac OS X software updates may change permissions on some files to improve security. When this occurs, the version of Disk Utility on the Mac OS X volume is updated to account for the new permissions. Running Disk Utility while started from the Mac OS X volume ensures that the changes made by software updates are preserved.
For much more on file permissions and repairs, also see this Apple article Troubleshooting permissions issues in Mac OS X