Handling "overlapped extent allocation" errors reported by Disk Utility or fsck

If Disk Utility or the fsck command line utility reports "overlapped extent allocation" errors that it cannot repair, here are some things you can do to resolve the issue. This affects both Mac OS X and Mac OS X Server. (This article offers advanced solutions that use the command line interface.)

About overlapped extent allocation errors

Overlapped extent allocation means that two files occupy the same space on your hard disk. Although this is a very rare situation, at least one of the affected files will likely have data loss. If Disk Utility or fsck reports this issue on your Mac OS X volume, do not install any additional software, but do try the solutions presented below.

Important: If you see an overlapped extent allocation error in Disk Utility or fsck, some data loss has already occurred on the disk. This article does not show how to recover lost data, but does provide steps on how to recover from this underlying issue. If you don't have a backup available and important information has potentially been lost, consider contacting a third-party data recovery service instead of attempting any of the steps below.

Backup, erase the volume, reinstall Mac OS X

The most effective way to resolve this issue is to back up as much important data as possible, start up from any Mac OS X Install or Restore disc, then perform an Erase and Install installation of Mac OS X (if this isn't an ideal solution for you, try the other options presented in this article).

Once you've reinstalled Mac OS X, restore or replace files from your recent backup (archive).

Tip: Making regular backups is a great way to prevent data loss.

Use Mac OS X 10.4.2 Disk Utility, or fsck, from a volume or disc

With Mac OS X 10.4.2 or later, Disk Utility can help you recover from these issues without erasing the volume. If your startup volume is affected, start up from a Mac OS X 10.4.2 or later disc or other 10.4.2 or later volume, and use Disk Utility to repair the affected volume (you can't use Disk Utility to repair while started from the affected volume).

Note: Only Mac OS Extended or Mac OS Extended (Journaled) volumes (also known as HFS Plus volumes, HFS Plus, HFS Plus Journaled, or HFSP Plus Case Sensitive) can be repaired this way.

If you don't have access to another Mac OS X 10.4.2 or later volume or Install disc ("version 10.4.2" or later appears on the disc's label), start up your computer in single-user mode and use fsck to repair the issues.

If you use Disk Utility on a Mac OS X 10.4.2 or later Install disc or fsck to repair the affected volume, you may be able to review some affected files using the "DamagedFiles" folder— see "Checking for files," below.

Important: If you do not have a Mac OS X 10.4.2 or later volume or Install disc that you can start up from, do not attempt to install the Mac OS X 10.4.2 Update, or any other software, on the affected volume at this point. Doing so may lead to further data loss. Instead, please use one of the other solutions presented below.

Checking for files--If you use Disk Utility or fsck in Mac OS X 10.4.2 or later to repair overlapped extent allocation issues, the following things happen when you repair an affected volume using one of these utilities:

• The repair utility attempts to move the data of the existing overlapped file (or files) to a new location on your hard disk. This move may succeed or fail without displaying any alert.
• If the move succeeds, the file gets updated to use the new data location. (You can move it again if you wish, of course.)
• If the move doesn't succeed because of a lack of contiguous available space or other reason, the file data is not moved.
• A new folder is created in the root level of your damaged hard disk named "DamagedFiles," which contains a symbolic link (or symlink, which is like an alias) with the name "fileID filename" for each file involved in the overlapping extents issue. Use the link (or links) to locate any affected files. Depending on the file, you can then determine if is still usable or if necessary, replace it with a backup. If the original file is a preference file, it's best to just delete it and recreate preferences from the associated application. Be sure to check the contents of all files listed in the DamagedFiles folder as well as in the output from Disk Utility or fsck. (Sometimes a file is created instead of a symlink; see below for details.)
  
  Important: The DamagedFiles folder may not always contain symlinks to all the files that were involved in producing the overlapped extents errors. You should write down the text output generated when you run Disk Utility or fsck with the actual symlinks or files in the DamagedFiles folder. You can also try to manually locate any files that were listed in Disk Utility or fsck error messages that don't appear in the DamagedFiles folder.
  
  Note: Any files in the DamagedFiles folder that have a fileID less than 16 are (or were) system files. Instead of a symlink, a plain text file gets created for these. Also, if an affected file's name has more than 255 characters, or the path to the file in the file system is greater than 1024 bytes, a plain text file gets created instead of a symlink—you can locate the original file manually to see if it's still usable. If the original file is a preference file, it's best to just delete it and recreate preferences from the associated application.

Try third-party disk repair software

You may also wish to try third-party disk utility applications, which may offer a resolution.

Use the command line to delete problematic files (advanced)

In some cases, removing the affected files and then recreating or restoring them from a backup may allow you to resolve the issue more easily, or at least to start up your computer so you can back up important files. Although these steps are typically for Mac OS X 10.4.1 or earlier, they can be used with Mac OS X 10.4.2 or later if other methods did not work.

Important: Follow the steps precisely. Incorrectly entered commands may result in data loss or unusable system software. Some steps involve using the rm command, which deletes files.

Before attempting the following steps, first consider trying the other solutions presented above.

1. Start up in single-user mode and use fsck.
2. Write down the file number for each overlapped extent allocation entry as it appears. For example, if you see this...
   
   ** Checking Catalog file.
   ** Overlapped extent allocation (file 5528909d)
   ** Overlapped extent allocation (file 9053642d)
   
   ...then write down "5528909" and "9053642" (ignore the trailing "d").
   
   Note: With Mac OS X 10.4.2 or later, the output appears like this:
   
   ** Overlapped extent allocation (file 1234567 /System/Library/Myfile.plist)
   
   
3. Find the path to each of the affected files. If only one or just a few files are listed, list each file path by entering this command for each affected file (where 5528909 is the number that fsck reported):
   
   find / -inum 5528909
   
   
4. Type mount -uw / (Then press Return)
5. Type rm and then a space (but don't press Return yet). The rm command is used to remove files or directories, so it's important that you follow these steps precisely.
6. On the same line, type the path to the affected file you located in step 3.
   
   Note: If a file's path contains a space (for example, /iTunes/iTunes Music/Random Song.m4a), type a backslash (\) before each space, or put the entire path in quotation marks. For example:
   
   /iTunes/iTunes\ Music/Random\ Song.m4a
   
   or
   
   "/iTunes/iTunes Music/Random Song.m4a"
   
   
7. Press Return. The file should be removed.
8. Repeat steps 5-7 for each affected file noted in step 2.
9. When finished, type reboot
10. Press Return.
11. Start up in single-user mode.
12. Type fsck -y (or fsck -fy for Mac OS Extended Journaled volumes).
    
    Important: If you are deleting flies to remove overlapping extents from your file system, it is essential that you perform this step (step 12). You must run fsck after deleting the overlapping files. If you don't, you may see a reoccurence of overlapping extent issues.
    
    
13. Press Return.
14. If the overlapped extent allocation errors still appear, repeat these steps.

After following these steps, restore or replace any removed files from a recent backup (or archive) if you have one. For a more thorough recovery from the issue, after backing up important files, perform an erase installation of Mac OS X.