Disk Utility's "First Aid" unreliable?

Discussion in 'macOS Mojave (10.14)' started by miretogo, May 11, 2019.

  1. miretogo macrumors member

    Aug 19, 2014
    Hello everyone,

    I installed macOS on a used iMac I bought and configured Time Machine. However, Time Machine backups failed to complete. Therefore, I used Disk Utility's "First Aid" to check if the external drive is ok and received error messages. I thought the drive was damaged. However, I partitioned the drive again and then everything worked and "First Aid" said the drive was ok.

    That got me thinking.

    A few weeks ago, I threw my old external hard drive in a box of electrics trash because I thought it was broken because Time Machine backups failed to complete and "First Aid" threw errors. Therefore, I picked it up from the trash pile, connected it and now suddenly everything works and the drive seems to be ok after all.

    1) Have you experienced something similar?
    2) How can I determine with confidence if a drive is ok or damaged if Disk Utility's "First Aid" seems unreliable?
  2. Honza1 macrumors 6502

    Nov 30, 2013
    FirstAid has been reliable for me. Also, seems errors are real as Time Machine has issues.
    Few other options to consider:
    1. Each run First aid actually attempts to fix the problems. I always un First aid few times. It may have been fixed during the first run.
    2. Any chance this is hardware issue? Cable, usb port, dock... something may be flaky.
  3. casperes1996 macrumors 68040


    Jan 26, 2014
    Horsens, Denmark
    First Aid errors aren't meant to tell you if the drive is dead. That was never the case. There could be a multitude of reasons for those errors. Drive errors are some yes, but unrepairable file system corruptions could also occur.

    If you want to know about a drive's health, keep an eye on its SMART status. But there's no definitive, conclusive way of predicting drive failure. It fails when it doesn't work at all anymore.
    --- Post Merged, May 11, 2019 ---
    Another point; There could also be certain sectors of the drive that are broken. If those sectors aren't touched it'll work perfectly fine, but anything touching those sectors won't.
  4. BLUEDOG314 macrumors 6502


    Dec 12, 2015
    IMO disk utility is next to useless. If you get an error that it can’t fix, just pull the data you can and wipe the drive.
  5. Honza1 macrumors 6502

    Nov 30, 2013
    The first sentence contradicts the second. useless = "not fulfilling or not expected to achieve the intended purpose or desired outcome"
    Disk Utility is very useful and needed. First Aid can fix some of the disk errors. Typically these would be errors caused by system crash or loss of power, accidental unmount of drive while being written to etc. In these cases First Aid can save data and effort needed to reinstall from backup. It was intended to fix these errors

    Also, without Disk Utility you will not be able to wipe the drive ;-) So it is useful already.

    There are some errors it cannot fix. Surely has no way of fixing hardware errors or results of hardware flakiness, etc. In that case restore from backup is the only way. But it is difficult to blame First Aid for that.

    Any tool has some limit to what it can do, and that is completely normal and should be expected. Over the last 10+ years I would guess First Aid was successful in ~90% cases for me. That is far from useless.
  6. KALLT macrumors 601

    Sep 23, 2008
    It does what it’s supposed to (as ‘first aid’ implies). It’s just a file-system check with limited repair options. If it can’t repair the issues it finds, it will recommend to reformat the disk as well.
  7. ApfelKuchen macrumors 68030

    Aug 28, 2012
    Between the coasts
    As others have noted, disk errors occur for a variety of reasons.

    The most common errors are not due to physical failure of the drive, but due to damaged file system records. The file system is the master catalog that records the physical location of every bit and byte on the drive. If that catalog is damaged, there's stuff that can't be found. If the catalog is too badly damaged, the only solution would be to erase/reformat the drive and create a new catalog from scratch.

    There will be times when the physical hard drive is failing in some way, but even then, as disks keep track of identifiably (physically) bad sectors, a reformat may still keep that drive running a while longer.
  8. CTHarrryH macrumors 68000

    Jul 4, 2012
    As others have stated there are file system errors that cause Time Machine to fail but the drive is physically and technically OK.
  9. miretogo thread starter macrumors member

    Aug 19, 2014
    Thanks for your replies. While I find it unsatisfactory that there seems to be no perfect method to check if a drive is okay or failing, I at least learned something new.
  10. NoBoMac, May 12, 2019
    Last edited: May 12, 2019

    NoBoMac macrumors 68020

    Jul 1, 2014
    Going to add: DU has worked well for me. And rarely need to run it for actual repairs. Do run it once a month on the external drives for sanity checks.

    Only time I've seen issues to repair is when the wrong plug is pulled from the hub (aka not ejecting the drive properly).

    And never shut the machine via pressing power button, always :apple: > Shutdown.

    Treat drives nicely, should not see issues.
  11. ApfelKuchen macrumors 68030

    Aug 28, 2012
    Between the coasts
    Why should there be a "perfect" way to detect potential drive failure, when there's no perfect way to predict the future?

    A disk utility like Disk Utility will detect existing disk directory damage. SMART is capable of detecting some kinds of physical performance degradation. But in the end, neither can detect something that hasn't happened yet. That's the main reasons we make backups - bad things can happen, and they often can't be predicted.

    One of the easiest predictors of equipment failure is time. The older the gear is, the more likely it will fail, especially when its parts are subject to mechanical wear. It's a lot like life. Eventually, we all will die. Knowing when is the real trick.
  12. miretogo thread starter macrumors member

    Aug 19, 2014
    Quite philosophical ;-) However, I did not mean predicting future failure but detecting existing damage.

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