That's a write permissions error. Check the permissions for the file in question.
Mac OS X incorporates a subsystem based on a UNIX-style operating system that uses permissions in the file system. Every file and folder on your hard disk has an associated set of permissions that determines who can read, write to, or execute it. Using the AppleWorks application and one of its documents as an example, this is what the permissions mean:
* Read (r--)
You can open an AppleWorks document if you have the read permission for it.
* Write (-w-)
You can save changes to an AppleWorks document if you have the write permission for it.
* Execute (--x)
You can open the AppleWorks application if you have the execute permission for it.
Also note that you must have execute permission for any folder that you can open; thus File Sharing requires execute permission set for other, world, and everyone for the ~/Public folder, while Web Sharing requires the same setting for the ~/Sites folder.
When you can do all three, you have "rwx" permission. Permissions for a folder behave similarly. With read-only permission to a folder containing documents, you can open and read documents but not save changes or add new documents to the folder. Read-only (r--) permission is common for sharing files with guest access, for example.
