“Underallocation Detected On Main Device” On macOS High Sierra…. What Is This Error?

The main reason I ended up replacing my the SSD in my wife’s MacBook Pro, which then led me to replace the battery as it was swelling and potentially dangerous was that after she upgraded to macOS High Sierra, she got this error message when disk utility was run on her MacBook Pro:


The error in question is “Underallocation Detected on Main device.” Now doing a Google search didn’t turn up any results in terms of what that error means. Though I did find a few reports of people having the same error. But if I had to take a guess, I would guess that when the High Sierra installer made the conversion from the old HFS+ filesystem to the new APFS filesystem, something went wrong and it didn’t properly allocate the space on the SSD. Now I may be wrong about that so if someone knows for sure what this error is, please reach out to me and let me know.

In the meantime, I needed to fix this. Since Disk Utility which is part of macOS High Sierra couldn’t fix this, and to my knowledge there are no other APFS compatible disk utilities out there at the moment, my plan of action was to:

  • Take a Samsung 850 EVO Pro 512GB SSD that I had lying around, place it inside this drive dock and connect it to the MacBook Pro via USB 3
  • Format the 850 EVO Pro for APFS without encryption
  • Use Carbon Copy Cloner to clone the drive inside the MacBook Pro to that drive,
  • Shut down the MacBook Pro upon completion of the cloning process and then swap the drive.
  • Fire it up to test that things worked fine hardware wise.
  • Run Disk Utility to confirm that the issues that sparked this were gone.
  • Check to see if TRIM was enabled for the drive (It was. But if it wasn’t I could use these instructions to enable it)
  • Reactivate Microsoft Office, rename the new SSD to “Macintosh HD” from “Untitled” and fix the Carbon Copy Cloner backup tasks that were still referencing the old drive.
  • Encrypt the drive

Now all of the above went to plan with the exception of the swelling battery. Had that not happened, it would have taken 2.5 hours to do everything but the encryption (which took about 14 hours to do by the way). But this is a pretty extreme way to solve this problem which I cannot imagine that the average user would do or could do. Plus, the only reason that I discovered this was that I ran Disk Utility a few days after the install to ensure that everything was fine, which in this case it wasn’t. Thus I have to wonder what would have happened if this was left unchecked. So I am wondering via this blog post if there was a different path I could have taken to address this? If anyone has any ideas, please leave a comment and let me know.


8 Responses to ““Underallocation Detected On Main Device” On macOS High Sierra…. What Is This Error?”

  1. […] part of fixing this problem with my wife’s MacBook Pro, which led me to fixing an even more serious problem, I replaced […]

  2. […] I had to fix an issue with my wife’s MacBook Pro by backing up her data, formatting her drive, and rest…. But that resulted in several people e-mailing me to ask a question. When I format the drive using […]

  3. […] every Mac that I have upgraded to High Sierra has run into an issue where there has been “underallocation” or “overallocation” of the APFS volume. This I discovered by using Disk Utility after the upgrade process has completed. Again, Apple has […]

  4. Andrew Illingworth Says:

    Excellent Blog, I have this problem – the underallocated thing. Noticed that after installing High Sierra FMP became unstable. Dis a disk check with Disk Utls, which as you point out can’t fix it.

    I your opinion could this be the cause of application instability. It seems to only be FMP that has become unstable?

    • I think that in theory any application that really ties into the file system could have issues. Having said that, I didn’t notice anything amiss until I ran the disk check. Thus I may be wrong on that front.

  5. Andrew Illingworth Says:

    Supplementary question – If I use Time Machine to restore to a pre High Sierra date, will doing that put the SSD back to as it was?

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