How can I repair a disk that Disk Utility says it can't repair?
There is a chance to repair it with command line (in Terminal) with the terminal utility as described in Apple's support docs, shown below:


  Use the command line and the fsck_hfs -l command.
  
  Start up your computer and log in as an administrator.
  
  Open Terminal (/Applications/Utilities).
  
  At the prompt, type the following command and then press Return to determine your filesytem ID:

df -hl

  
  Look for some lines of text that look like this:

Filesystem Size Used Avail Capacity Mounted on
/dev/disk0s3 37G 20G 17G 55% /
/dev/disk0s5 37G 37G 641M 98% /Volumes/Storage

  
  Make a note of the External Disk "disk" name that appears after /dev/, such as "/dev/disk0s3." This is your filesystem ID for your External volume.
  
  At the prompt, type the following command and then press Return:

df -hl

  
  Then type the following command, where "disk1" is your filesystem ID you noted in step 4, then press Return:

sudo fsck_hfs -l /dev/disk1

  
  When prompted, enter your admin password, then press Return to begin the verification.
  
  You should see messages like these during the disk check:

** /dev/rdisk0s3 (NO WRITE)
** Root file system
** Checking HFS Plus volume.
** Checking Extents Overflow file.
** Checking Catalog file.
** Checking multi-linked files.
** Checking Catalog hierarchy.
** Checking Extended Attributes file.
** Checking volume bitmap.
** Checking volume information.
** The volume Macintosh HD appears to be OK.