Disk could not be partitioned - potential disk errors

Loner T wrote:

In Single-User Mode (or Internet Recovery), try to run

fsck_apfs -s -o -y /dev/rdisk1

Also, as a test, create FAT partition using Disk Utility of the size you want Windows to be. This can be removed later.

Thanks for your help.

'fsck_apfs -s -o -y /dev/rdisk1' in Single User Mode results in 'error: container /dev/rdisk1 is mounted. repairs in a mounted container is not supported yet.'

Trying to create a FAT partition fails as below:

Partitioning disk “APPLE SSD SM1024F Media” (disk0)

Resizing the startup volume will cause this computer to stop responding.

Running operation 1 of 2: Add “Test” 100.46 GB by shrinking container disk1 “HD” (disk0s2) from 1 TB to 899.89 GB…
Aligning shrink delta to 100,457,570,304 bytes and targeting a new physical store size of 899,888,254,976 bytes
Determined the minimum size for the targeted physical store of this APFS Container to be 241,390,583,808 bytes
Resizing APFS Container designated by APFS Container Reference disk1
The specific APFS Physical Store being resized is disk0s2
Verifying storage system
Using live mode.
Performing fsck_apfs -n -x -l /dev/disk0s2
Checking the container superblock.
Checking the EFI jumpstart record.
Checking the space manager.
Checking the space manager free queue trees.
Checking the object map.
Checking volume.
Checking the APFS volume superblock.
The volume HD - Data was formatted by hfs_convert (748.1.46) and last modified by apfs_kext (1677.60.23).
Checking the object map.
Checking the snapshot metadata tree.
Checking the snapshot metadata.
Checking the extent ref tree.
Checking the fsroot tree.
error: Cross Check : FSroot tree references extent (0xdfdca73 + 88) which is not present in the ExtentRef tree
error: Cross Check : Mismatch between extentref entry reference count (0) and calculated fsroot entry reference count (1) for extent (0xdfdca73 + 88)
Checking volume.
Checking the APFS volume superblock.
The volume Preboot was formatted by newfs_apfs (748.1.46) and last modified by apfs_kext (1677.60.23).
Checking the object map.
Checking the snapshot metadata tree.
Checking the snapshot metadata.
Checking the extent ref tree.
Checking the fsroot tree.
Checking volume.
Checking the APFS volume superblock.
The volume Recovery was formatted by newfs_apfs (748.1.46) and last modified by apfs_kext (1677.60.23).
Checking the object map.
Checking the snapshot metadata tree.
Checking the snapshot metadata.
Checking the extent ref tree.
Checking the fsroot tree.
Checking volume.
Checking the APFS volume superblock.
The volume VM was formatted by newfs_apfs (748.1.46) and last modified by apfs_kext (1677.60.23).
Checking the object map.
Checking the snapshot metadata tree.
Checking the snapshot metadata.
Checking the extent ref tree.
Checking the fsroot tree.
Checking volume.
Checking the APFS volume superblock.
The volume HD was formatted by storagekitd (1677.40.17.161.3) and last modified by apfs_kext (1677.60.23).
Checking the object map.
Checking the snapshot metadata tree.
Checking the snapshot metadata.
Checking snapshot 1 of 1 (com.apple.os.update-5523D8E63431315F9F949CCDD0274BF797F5CEE4EAF616D4C66A01B8D6A83C7B)
Checking the extent ref tree.
Checking the fsroot tree.
Checking the file extent tree.
Checking volume.
Checking the APFS volume superblock.
The volume Update was formatted by com.apple.Mo (1677.40.17.161.3) and last modified by apfs_kext (1677.60.23).
Checking the object map.
Checking the snapshot metadata tree.
Checking the snapshot metadata.
Checking the extent ref tree.
Checking the fsroot tree.
Verifying allocated space.
warning: overallocation detected on Main device: (0xdfdca73+88) bitmap address (0x6b77b)
The volume /dev/disk0s2 appears to be OK.
Storage system check exit code is 0.
Shrinking APFS Physical Store disk0s2 from 1,000,345,825,280 to 899,888,254,976 bytes
Shrinking APFS data structures
APFS Container Resize error code is 49174
A problem occurred while resizing APFS Container structures. : (-69606)

Operation failed…

Can you unmount the disk and try to run the fsck_apfs? If that does not work, boot into Internet Recovery, and try to run the same.

I was unable to unmount the disk in Single User (something about disk arbitration unavailable in that mode) or Internet Recovery (unmount dissented by PID 0 kernel task). Otherwise same message about mounted containers in both modes.

This does indicate APFS corruption. Do you have a Time Machine backup?

I do indeed have a Time Machine backup.

That was indeed the only way to fix this which was a hassle. Which must mean it was APFS corruption of some kind.

Erasing and restoring allowed partitioning and bootcamp install.

Thanks for your help!