Recently, my Mid 2012 13-inch MacBook Pro displayed a blank, gray screen instead of booting up. I suspected there was a problem with the hard drive cable, so I took it in to a (non-Apple) repair shop to get a repair quote.
It turns out there was a problem with the cable, but they also said there was a problem with the hard drive. They said their tests revealed a “cyclic redundancy check” or CRC error which would necessitate total replacement of the hard drive.
I then took the MacBook apart, removed the drive, plugged it into another Mac with a usb-adapter and ran First Aid in Disk Utility.
Now, disk utility tells me there’s nothing wrong with the drive (big green check next to drive icon and no error messages.) I can also explore it in finder without issue.
So, my question is this: is it that disk utility does not recognize CRC errors where they exist, or is my hard drive actually fine and the repair shop people were wrong?
Best Answer
The test in Disk Utility really only tests the file system - i.e. the high level structure of the disk’s organization into files, folders, metadata, etc. Structural errors would show up in Disk Utility.
If your file system is based on HFS+, there’s no CRC checksum or any other type of checksum at the file system layer.
If you’re running High Sierra or Mojave and have been upgraded to APFS, then there’s a checksum of the file system metadata. It’s not CRC though.
What Disk Utility doesn’t really check is your actual data - i.e. file contents. As neither HFS+ nor APFS stores checksums of data, it cannot be checked.
This means that either the repair shop used other software to store and retrieve data on the drive, and this failed CRC checks - or they’re talking about CRC errors on the SATA bus. The latter would be expected with a bad cable - and should then be fully resolved by replacing the cable - data might have been lost, but the disk isn’t broken.