Summary: 'File system verify or repair failed' in Disk Utility, along with 'File system check exit code is 8', usually occurs to external hard drives, USB flash drives, SD cards, SSD, as well as Mac internal hard drives. Given that code is a series literally 10,000,000x strings of 1 and 0's things do go astray. Exit code 8 is a generic failure where a special character cannot be read, or a character is unexpectedly in the wrong place. If you have access to an old mac, this may be a quicker way to get at your files than DiskWarrior, etc. After failing in Yosemite, the same WD drive works fine in OS 10.6.8. I've experienced the same problems described here (exit code 8).
Even though Mac OS X is considered as one of the stable operating systems, sometimes, even normally functioning Mac mac hine may show up a sudden warning message that says “Mac hard drive exit code 8” or “file system check exit code is 8”. The ‘File system check exit code is 8’ or ‘Mac hard drive exit code 8’ errors occur mainly at the time of system check and is capable of taking place in both internal and external hard drives.