By default OS X is set up so you can access and edit files you create and only requires authentication to edit files in system directories or in other user accounts (provided you are an administrator). This allows for a fairly seamless workflow when working with the resources your account has access to, and notifies you when you are editing files that could affect other users or the system as a whole.
This setup is determined by the permissions settings in the system, which you can see if you select a file and get information on it. At the bottom of the information window you will see a Sharing & Permissions section with a list of names and privileges. By default the topmost name is the user or account that created it, which could be your account or the system. The second name is the group associated with the file, such as "admin" to include all administrators, or "staff" to include all user accounts on the system, among others. The last name should be "everyone," which describes the access permissions for all other users or groups defined in the system, including guest accounts.
If a problem happens with the permissions setup on the system, you may run into errors when you access or edit files, including the need to enter your admin password anytime you wish to move, open, or edit a specific file (even those in your user account). These errors usually do not happen just by using the system, and instead are often a byproduct of restoring your system from a backup, migrating to a new system, or performing some systemwide modification or tweaking routine.
If you are experiencing permissions errors and getting these warnings when editing files, you can take several approaches to clearing the problem. The approach that's best, though, depends on the exact nature of the issue.
- Happening everywhere
Another area where permissions problems might seem to be happening everywhere is if they are rampant through your home directory, which can happen if you have copied files to your system from a backup, migration, or via Target Disk mode. Unfortunately Disk Utility's permissions fix will not address the permissions in your home folder; however, Apple does have another way to fix these issues.
If you are using OS X Lion, reboot to the recovery partition by holding Command-R at startup, or otherwise boot to your OS X installation DVD. When in the OS X installer, in versions prior to Lion select Reset Password from the Utilities menu, and in Lion select the Terminal and type "resetpassword" to open this same utility.
With the utility open, select your hard drive and your account from the pop-up menu, followed by clicking the button to Reset Home Directory Permissions and ACLs. After this routine is performed, reboot the system and see if you can access your files again.
- Happening for one file or folder
If you are only having problems accessing one folder, such as one you extracted from a ZIP archive, or one you copied from another computer, then you do not have to address the problem by resetting the permissions for all files on your system. Instead, you can tackle those for the file or directory itself. To do this, select the item and get information on it by pressing Command-I on the keyboard. Click the lock at the bottom of the window and authenticate, followed by selecting and removing the username and group from the Sharing & Permissions list. Then click the plus button at the bottom of the window and add your user account to the list (it should say "me" next to it), and ensure that your account has both Read and Write permisisons.
- Happening on external hard drives
By default OS X is set to manage permissions on the boot drive, since external drives can be attached to other system where files can inherit odd settings that might affect how the file is accessed in OS X. While you can use the second method mentioned above to address problems on external drives, another approach is to have the system simply ignore permissions on these drives. To do this, select the drive in the Finder and get information on it, followed by checking the option to Ignore ownership on this volume at the bottom of the window.