I have a 2018 Macbook Pro that I recently started updating from Mojave to Big Sur.

I received an error message when the installation was finishing up.

"An error occurred preparing the software update."

After further googling it seems that unfortunately there was not enough HD space to complete the installation and MacOS did nothing to warn me of this before starting the installation, and let me do it when there clearly wasn't enough HD space free. Only 20mb is currently remaining on the drive.

I'm now in a situation where I can't boot into any form of desktop. I realise I'll probably have to format the drive and do a fresh install, but I'd ideally like to recover some files from the desktop folder before doing this.

I then connected the Macbook to my older iMac via Thunderbolt and put the Macbook into Target Disk Mode, but for some very strange reason after a (lengthy period of waiting) only the SYSTEM folder appears in the mounted drive. I'm unable to see any other folder for some reason. The USERS folder is nowhere to be seen.

I then thought if I created an install of Big Sur on an external drive, I could boot from that and then from the desktop I could access the USERS folder on the internal drive in order to rescue my files.

I created the installation on the external drive, but when the computer restarted and automatically went to boot from that drive, I received the following ""Authentication Needed - You will need to authenticate as an Administrator to change the boot security settings."

When I click on Enter macOS Password, I get this error : "Recovery is trying to change system settings. No administrator was found." So it doesn't even give me the option to enter the password.

This is very strange to me, as I have admin access to this laptop, but it's not even allowing me to enter anything.

I've looked into a few things now, and it seems these newer macs have some Secure Token on the T2 chips, and this error could be linked to that, but I'm not 100% on this.

It's worth mentioning at this point that the computer is a company laptop and after speaking to our IT department they assure me that they certainly don't setup any firmware passwords or encryption at a boot level, and they don't select any options that prevent the mac from booting from an external drive.

Their only solution at this point is a format and reinstall, but I'm sure there must be another way around this.

Does anyone have any advice on how I can access these files before formatting?

Or even freeing up enough HD space so Big Sur can complete the installation?

Is there a way to allow the mac to boot from an external drive given the error message "Recovery is trying to change system settings. No administrator was found."

A way to bypass this using Terminal perhaps?

Or even, somehow copy folders to an external device via Terminal when the mac is in recovery mode.

Any advice or things to try would be much appreciated.

Thanks so much

diskutil list shows:

/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *251.0 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                 Apple_APFS Container disk2         250.8 GB   disk0s2

/dev/disk1 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *128.0 GB   disk1
   1:                        EFI EFI                     209.7 MB   disk1s1
   2:                  Apple_HFS Little SSD              127.7 GB   disk1s2

/dev/disk2 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +250.8 GB   disk2
                                 Physical Store disk0s2
   1:                APFS Volume Big SSD Boot            226.3 GB   disk2s1
   2:                APFS Volume Preboot                 21.8 MB    disk2s2
   3:                APFS Volume Recovery                516.1 MB   disk2s3
   4:                APFS Volume VM                      1.1 GB     disk2s4

/dev/disk3 (external):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                         500.3 GB   disk3
   1:                        EFI EFI                     314.6 MB   disk3s1
   2:                 Apple_APFS Container disk4         500.0 GB   disk3s2

/dev/disk4 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +500.0 GB   disk4
                                 Physical Store disk3s2
   1:                APFS Volume Macintosh HD — Data     495.5 GB   disk4s1
   2:                APFS Volume Preboot                 111.7 MB   disk4s2

diskutil apfs list shows:

>APFS Containers (2 found)
+-- Container disk2 18FC179D-FDA1-4536-A4D5-C0892469C525
|   ====================================================
|   APFS Container Reference:     disk2
|   Size (Capacity Ceiling):      250790436864 B (250.8 GB)
|   Minimum Size:                 235494641664 B (235.5 GB)
|   Capacity In Use By Volumes:   228006752256 B (228.0 GB) (90.9% used)
|   Capacity Not Allocated:       22783684608 B (22.8 GB) (9.1% free)
|   |
|   +-< Physical Store disk0s2 69CB5811-EB1F-4365-89B8-AE4D6585D772
|   |   -----------------------------------------------------------
|   |   APFS Physical Store Disk:   disk0s2
|   |   Size:                       250790436864 B (250.8 GB)
|   |
|   +-> Volume disk2s1 1389C234-C0BD-341E-AE08-F4ED034A8EF0
|   |   ---------------------------------------------------
|   |   APFS Volume Disk (Role):   disk2s1 (No specific role)
|   |   Name:                      Big SSD Boot (Case-insensitive)
|   |   Mount Point:               /
|   |   Capacity Consumed:         226256613376 B (226.3 GB)
|   |   FileVault:                 No
|   |
|   +-> Volume disk2s2 FDBA5B37-70F8-4CC9-ABE5-BC70408BD83F
|   |   ---------------------------------------------------
|   |   APFS Volume Disk (Role):   disk2s2 (Preboot)
|   |   Name:                      Preboot (Case-insensitive)
|   |   Mount Point:               Not Mounted
|   |   Capacity Consumed:         21762048 B (21.8 MB)
|   |   FileVault:                 No
|   |
|   +-> Volume disk2s3 3B02E4D1-D0A1-486D-951A-F7CAC8DEAC4B
|   |   ---------------------------------------------------
|   |   APFS Volume Disk (Role):   disk2s3 (Recovery)
|   |   Name:                      Recovery (Case-insensitive)
|   |   Mount Point:               Not Mounted
|   |   Capacity Consumed:         516112384 B (516.1 MB)
|   |   FileVault:                 No
|   |
|   +-> Volume disk2s4 5A1EEF42-0512-48F9-8928-FAACC6BA0DB2
|       ---------------------------------------------------
|       APFS Volume Disk (Role):   disk2s4 (VM)
|       Name:                      VM (Case-insensitive)
|       Mount Point:               /private/var/vm
|       Capacity Consumed:         1073897472 B (1.1 GB)
|       FileVault:                 No
+-- Container disk4 5349EC5A-7FC0-4FE6-8B90-9D53B64EE819
    APFS Container Reference:     disk4
    Size (Capacity Ceiling):      499963174912 B (500.0 GB)
    Minimum Size:                 499963174912 B (500.0 GB)
    Capacity In Use By Volumes:   499942440960 B (499.9 GB) (100.0% used)
    Capacity Not Allocated:       20733952 B (20.7 MB) (0.0% free)
    +-< Physical Store disk3s2 EDBCB48D-D8CD-44FB-AD64-570ACE75C4E1
    |   -----------------------------------------------------------
    |   APFS Physical Store Disk:   disk3s2
    |   Size:                       499963174912 B (500.0 GB)
    +-> Volume disk4s1 387E5B2F-8741-4B94-9C31-75C01085406C
    |   ---------------------------------------------------
    |   APFS Volume Disk (Role):   disk4s1 (No specific role)
    |   Name:                      Macintosh HD — Data (Case-insensitive)
    |   Mount Point:               Not Mounted
    |   Capacity Consumed:         495465476096 B (495.5 GB)
    |   Encrypted:                 ERROR -69808
    +-> Volume disk4s2 C1185C96-8408-43A8-B158-A661D54BC8EE
    |   ---------------------------------------------------

Update Results of file system verification:

Started file system verification on disk4s1 Macintosh HD — Data

Verifying file system

Volume is already unmounted

Live mode required because other APFS Volumes in its Container are mounted

Using live mode

Performing fsck_apfs -n -l -x /dev/rdisk4s1

Checking volume

Checking the container superblock

Checking the EFI jumpstart record

Checking the space manager

Checking the object map

Checking the APFS volume superblock

warning: apfs_sb at apfs_fs_index (0): apfs_features has unrecognized features (10)

Checking the object map

error: mount_apfs exit status 73

The volume /dev/rdisk4s1 could not be verified completely

File system check exit code is 78

Restoring the original state found as unmounted

Error: -69845: File system verify or repair failed

Underlying error: 78: Function not implemented


It's almost certainly the bug that you referenced here. https://mrmacintosh.com/big-sur-upgrade-not-enough-hd-space-serious-issue-possible-data-loss/

Although both recovery options to save the data requires the macbook being mountable via TDM and I'm unable to do this as it's greyed out and not mounting in disk utility. What would you suggest as a next step?

  • I assume you can boot to Big Sur internet recovery. Open a terminal window and copy files to an external drive. Commented Feb 9, 2021 at 3:31
  • I can yes, I haven't tried accessing any files in terminal yet, just as I'm unsure of the syntax I need to copy files from the internal drive to the external drive. I've used Terminal before, but only copy and pasting commands from advice on forums etc. Commented Feb 9, 2021 at 3:36
  • Please use the blockquote style in the editor for pasting output from Terminal to make it easier to read.
    – pion
    Commented Feb 9, 2021 at 11:03
  • Also your diskutil apfs list invocation got truncated. You may way to use PasteBin.com if it’s too long to paste.
    – pion
    Commented Feb 9, 2021 at 11:15
  • (Another request to please use blockquote or code formatting [“{ }” icon] when pasting Terminal output to Stack Exchange, as it’s very difficult to read otherwise. You can refer to Stack Exchange help if you’re having trouble with this.)
    – pion
    Commented Feb 9, 2021 at 11:32

1 Answer 1


You aren’t seeing the Users directory because of the read-only system volume that was introduced in Catalina: For every system volume named “Foo” there is a second volume named “Foo - Data” that contains your Users directory.

You mentioned that you can boot to Target Disk Mode. If that is still the case, go ahead and do that, but this time on your lifeboat Mac open Disk Utility, go to View -> Show All Devices, and check to see if there is an unmounted (greyed-out) volume next to your target’s “system” volume which has the “- Data” suffix. If so, select it and press the Mount button. You may need to enter a FileVault password at this point.

Alternatively, you can use Terminal on your lifeboat to examine which volumes are available and what their statuses are:

$ diskutil list
$ diskutil apfs list

You can also do the same thing when booted into Recovery mode (Tools -> Terminal).


With your target in TDM and mounted on your lifeboat, run the following. (Make sure not to disconnect your target or add/remove any other devices from your lifeboat, otherwise the disk numbers may change and your data volume might not be on /dev/disk4s1.)

$ sudo diskutil verifyVolume /dev/disk4s1

UPDATE 2: As I keep seeing more and more reports like this, I suspect that you are also in the camp hit with the unfortunate Big Sur installation bug. I suggest reading through that before continuing to debug.

  • I hooked the macbook back up to the iMac in target disk mode and loaded disk utility. For some reason, it takes 5 mins to be able to show the two external volumes. I assume because of the lack of hard disk space, I have Macintosh HD and Update showing. SHOW ALL DEVICES was already enabled by default Now using Terminal on the iMac. I have navigated my way to /volumes and i see: Internal 1, Macintosh HD (external) and Update (external) then going cd 'Macintosh HD' shows me "System, bin, dev, sbin, usr." I can't see users anywhere? Do I need to unhide the data volume you mentioned? Commented Feb 9, 2021 at 10:40
  • Please run the diskutil commands I mentioned above.
    – pion
    Commented Feb 9, 2021 at 10:41
  • Please add all updates to your original question, not in comments
    – pion
    Commented Feb 9, 2021 at 10:56
  • In the meantime, the greyed out Macintosh HD — Data external drive has appeared in Disk Utility, but clicking mount does nothing. Commented Feb 9, 2021 at 11:04
  • 1
    @cocktailsanddreams Did you end up formatting and reinstalling?
    – pion
    Commented Mar 11, 2021 at 4:28

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