Tuesday, July 10, 2012

How to Resolve 'Overlapped Extent Allocation' Issue in Mac OS X

Apple’s Disk Utility can perform a range of disk-related tasks in Mac OS X. This inbuilt tool allows verifying a disk’s identity. If errors are found, it repairs the disk to fix the damage. You may come across the following error while performing a repair using Disk Utility:

‘overlapped extent allocation’

The term ‘overlapped extent allocation’ implies that two files are stored on the same area of your hard disk. This condition indicates that you have already lost some of your data stored on the disk. You may try to fix the issue using any of the following methods:

  • An ideal solution is to back up all valuable data, boot from your Mac OS X Install disc and then carry out an ‘Erase and Install’ installation of Mac OS X. After a successful installation, you should restore data from the most recent backup. If this does not work for you, try other methods.
  • You can use the Disk Utility of Mac OS X 10.4.2 or later to repair the corrupt disk. In case of startup disk, you may need to boot from a Mac OS X 10.4.2 or later Install disc or any other volume of your Mac OS X 10.4.2 or later. This method can be only used for repairing Mac OS Extended or Mac OS Extended (Journaled) volumes.     
  • If you do not have Mac OS X 10.4.2 or later Install disc, you can boot your Mac in single-user mode and run ‘fsck’ to repair disk corruption.

In case you use any of the aforementioned repair utilities in Mac OS X 10.4.2 or later, the following things occur:

  • All data stored in the existing overlapped file is transferred to a different location on the disk. You may not receive any alert if this operation fails.
  • If the transfer is performed successfully, the overlapped file starts using the new data location.
  • If the transfer fails (due to insufficient contiguous free space), the file data remain at the same location.

You can identify the files that are causing this problem by checking the ‘DamagedFiles’ folder created in the root level of your hard disk. You can replace or restore these files from a backup.

If you still receive the same error on running Disk Utility, take help of professional Mac Data Recovery software. These advanced utilities allow recovering all lost, deleted, or formatted data from any HFS, HFS+, and FAT file system based drives or volumes. Furthermore, they support Mac OS X 10.7 Lion.

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