If you have attempted to partition a drive from Mac OS X and received a “Partition failed” message with the error “Couldn’t modify partition map because file system verification failed.” you can fix the problem with the file system check command line utility.

To issue the file system check and repair function properly, you’ll need to boot into Single User Mode as well. This walkthrough will demonstrate the necessary steps to resolve the partition failed error by running a file system command from Mac Single User mode.

Fixing “Partition Failed” Errors in Mac OS X

Be sure you back up your Mac before beginning.

Reboot the Mac into Single User Mode by holding Command+S during boot At the command prompt, type: /sbin/fsck -fy When fsck is complete, type “exit” or “reboot” to reboot Boot the Mac as usual, verify the disk again in Disk Utility, and partition as usual

Boot the Mac as usual and relaunch Disk Utility to verify the disk.

At this point, you can go ahead and partition the drive as usual, this time without any such “Partition Failed” error message appearing in Disk Utility.

Often users may see this error when attempting to partition a Mac boot drive for dual booting or for some similar purpose. I have encountered this error a few times when partitioning the boot drive from OS X Lion, most recently after a clean installation when setting up dual boot for OS X Lion and Mountain Lion. The cause of this remains to be seen, and repairing the disk from Disk Utility itself doesn’t work, even when in Single User Mode or when booting from another drive. But, fortunately fsck does work just fine for most situations where ‘partition failed’ error is showing up in Mac OS, so give it a try.

Do note that “fsck” can take a while to run and complete, depending on the size of the drive.

You can also use the ‘fsck_hfs’ command if desired.

If you have another solution to a partition failed error in Mac OS Disk Utility, share with us in the comments below.