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User profile for user: schrepfler
schrepfler Author
User level: Level 1
12 points

Underallocation Detected on Main device

The upgrade to High Sierra didn’t go quite as I expected. After leaving the computer overnight (MBP, later 2013, 500GB SSD) and a day it got stuck on the Apple loading menu.

After a reset the computer kept hanging at about 90%. After smc reset, safe mode, and restore mode os reinstall as well as a disk First Aid, no luck. 😭

fsck_apfs returns “Underallocation Detected on Main device”, attaching screenshot.

Is there any way this can be fixed please? No TimeMachine either. I can see Macintosh HD mounted in restore mode with around 80 GB free, can I somehow save the data/profile if there’s no other way than format the partitions?


User uploaded file

Posted on Sep 28, 2017 1:42 PM

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Question marked as Top-ranking reply
User profile for user: sterling r
sterling r
User level: Community Specialist

Posted on Oct 5, 2017 10:42 AM

Hi schrepfler,

I understand that when you upgraded to macOS High Sierra your Mac was stuck on the boot screen with the Apple logo. I also see that after restarting, attempting safe mode, an SMC reset, First Aid via Disk Utility and a software reinstall you're seeing an "Underallocation Detected on Main device" message in First Aid. I know it's important for your Mac to work properly and I'm happy to assist you. Based on the information you've provided, the next step will be to erase and reinstall macOS. This will erase all personal data on your Mac, you had also mentioned "No Time Machine", so in order to attempt to save your information prior to erasing and reinstalling macOS please follow the steps below:

If Disk Utility can't repair your startup disk

If Disk Utility finds issues with your startup disk that it can't repair, you might need to reformat it. You should back up any important data on your startup disk before erasing it. Erasing your startup disk deletes everything stored on it, including items in your home folder like your documents and items on your desktop.

If you don't have a recent backup of personal data that's stored on your startup disk, you can use these steps to try to back up your data to an external drive:

  1. Connect an external USB, Thunderbolt, or FireWire drive to your Mac. The drive needs to be the same size or larger than your current startup disk. It also needs to be a drive that you can erase.
  2. Use macOS Recovery to erase the external drive, then install macOS onto the external drive. Make sure that you select the external disk as the one you want to erase. Don't select your built-in startup disk, usually named Macintosh HD.
  3. After installation is finished, your Mac automatically restarts from the external drive. When Setup Assistant appears, select the option to migrate your data from another disk. Choose your built-in startup disk as the source to migrate your data from.
  4. When migration is done, complete the steps of the setup assistant. After the desktop appears, confirm that your data is present on the external drive.

Now that you've made a backup to an external drive, follow these steps:

  1. Use macOS Recovery to erase your built-in startup disk and reinstall macOS. Select the built-in startup disk as the one you want to erase, not the external drive. When installation is finished, your Mac automatically restarts from your built-in startup disk.
  2. After you erase your startup disk and reinstall macOS, your Mac automatically restarts and setup assistant appears. To copy your personal data back to your startup disk, select the option in the setup assistant to migrate your data from an existing Time Machine backup or other disk. Choose the external drive as the source to migrate your data from.

If you're unable to erase your startup disk or reinstall macOS, your Mac might need repair. Make an appointment with an Apple Genius or an Apple Authorized Service Provider for more help.

If a flashing question mark appears when you start your Mac


Best Regards.
View in context
2 replies
Question marked as Top-ranking reply
User profile for user: sterling r
sterling r
User level: Community Specialist

Oct 5, 2017 10:42 AM in response to schrepfler

Hi schrepfler,

I understand that when you upgraded to macOS High Sierra your Mac was stuck on the boot screen with the Apple logo. I also see that after restarting, attempting safe mode, an SMC reset, First Aid via Disk Utility and a software reinstall you're seeing an "Underallocation Detected on Main device" message in First Aid. I know it's important for your Mac to work properly and I'm happy to assist you. Based on the information you've provided, the next step will be to erase and reinstall macOS. This will erase all personal data on your Mac, you had also mentioned "No Time Machine", so in order to attempt to save your information prior to erasing and reinstalling macOS please follow the steps below:

If Disk Utility can't repair your startup disk

If Disk Utility finds issues with your startup disk that it can't repair, you might need to reformat it. You should back up any important data on your startup disk before erasing it. Erasing your startup disk deletes everything stored on it, including items in your home folder like your documents and items on your desktop.

If you don't have a recent backup of personal data that's stored on your startup disk, you can use these steps to try to back up your data to an external drive:

  1. Connect an external USB, Thunderbolt, or FireWire drive to your Mac. The drive needs to be the same size or larger than your current startup disk. It also needs to be a drive that you can erase.
  2. Use macOS Recovery to erase the external drive, then install macOS onto the external drive. Make sure that you select the external disk as the one you want to erase. Don't select your built-in startup disk, usually named Macintosh HD.
  3. After installation is finished, your Mac automatically restarts from the external drive. When Setup Assistant appears, select the option to migrate your data from another disk. Choose your built-in startup disk as the source to migrate your data from.
  4. When migration is done, complete the steps of the setup assistant. After the desktop appears, confirm that your data is present on the external drive.

Now that you've made a backup to an external drive, follow these steps:

  1. Use macOS Recovery to erase your built-in startup disk and reinstall macOS. Select the built-in startup disk as the one you want to erase, not the external drive. When installation is finished, your Mac automatically restarts from your built-in startup disk.
  2. After you erase your startup disk and reinstall macOS, your Mac automatically restarts and setup assistant appears. To copy your personal data back to your startup disk, select the option in the setup assistant to migrate your data from an existing Time Machine backup or other disk. Choose the external drive as the source to migrate your data from.

If you're unable to erase your startup disk or reinstall macOS, your Mac might need repair. Make an appointment with an Apple Genius or an Apple Authorized Service Provider for more help.

If a flashing question mark appears when you start your Mac


Best Regards.
Reply

Underallocation Detected on Main device

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