Boot Camp Assistant partition error— what to try next?

Hi, thanks for getting back to me. I should update to say I tried the snapshot-deleting approach above but with no luck.

Here is the output of sudo fsck_apfs -n -l /dev/rdisk1:

** 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 Macintosh HD - Data was formatted by hfs_convert (748.57.19) and last modified by apfs_kext (1677.50.1).

** 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 (0x686378d + 64) which is not present in the ExtentRef tree

error: Cross Check : FSroot tree references extent (0x686380d + 64) which is not present in the ExtentRef tree

error: Cross Check : FSroot tree references extent (0x68637cd + 64) 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 (0x686378d + 64)

error: Cross Check : Mismatch between extentref entry reference count (0) and calculated fsroot entry reference count (1) for extent (0x68637cd + 64)

error: Cross Check : Mismatch between extentref entry reference count (0) and calculated fsroot entry reference count (1) for extent (0x686380d + 64)

warning: apfs_fs_alloc_count is not valid (expected 66917071, actual 66917263)

Fix apfs_fs_alloc_count (oid 0x403, xid 0x5c84b4)? NO

** Checking volume.

** Checking the APFS volume superblock.

** The volume Preboot was formatted by newfs_apfs (748.57.19) and last modified by apfs_kext (1677.50.1).

** 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 (748.57.19) and last modified by apfs_kext (1677.50.1).

** 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 (748.57.19) and last modified by apfs_kext (1677.50.1).

** 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 Macintosh HD was formatted by storagekitd (1677.50.1) and last modified by apfs_kext (1677.50.1).

** Checking the object map.

** Checking the snapshot metadata tree.

** Checking the snapshot metadata.

** Checking snapshot 1 of 1 (com.apple.os.update-779BDF1556C6F688504E24FB29C75AFFABFCB91E701806FFFF35235E19914F1E)

** 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.Mobile (1677.50.1) and last modified by apfs_kext (1677.50.1).

** 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: (0x686378d+192) bitmap address (0x9db7)

Fix overallocation (0x686378d+192) bitmap address (0x9db7)? NO

** The volume /dev/rdisk1 appears to be OK.

Ok, thanks again, I’ll try that tomorrow. So I’m just booting into Internet Recovery and hitting fsck_apfs -s -o -y /dev/rdisk1 in Terminal? If you have time, could you please briefly explain (in layman’s terms, if possible) what’s going on here?

Do you mean click on and unmount Container disk1, rather than Macintosh HD? I thought Macintosh HD was the volume, not the container. Disk Utility (at least in normal mode) is also indicating that Macintosh HD is already unmounted.

So I’ve gone into Internet Recovery, unmounted Macintosh HD (Container disk1 no longer appears in Disk Utility) and run the fsck again, but I get the same error message. (container /dev/disk1 is mounted. repairs in a mounted container is not yet supported.)

/dev/disk0 (internal, physical):

   #:                       TYPE NAME                    SIZE       IDENTIFIER

   0:      GUID_partition_scheme                        *500.3 GB   disk0

   1:                        EFI ⁨EFI⁩                     209.7 MB   disk0s1

   2:                 Apple_APFS ⁨Container disk1⁩         500.1 GB   disk0s2

/dev/disk1 (synthesized):

   #:                       TYPE NAME                    SIZE       IDENTIFIER

   0:      APFS Container Scheme -                      +500.1 GB   disk1

                                 Physical Store disk0s2

   1:                APFS Volume ⁨Macintosh HD - Data⁩     288.7 GB   disk1s1

   2:                APFS Volume ⁨Preboot⁩                 281.3 MB   disk1s2

   3:                APFS Volume ⁨Recovery⁩                655.5 MB   disk1s3

   4:                APFS Volume ⁨VM⁩                      2.1 GB     disk1s4

   5:                APFS Volume ⁨Macintosh HD⁩            14.9 GB    disk1s5

   6:              APFS Snapshot ⁨com.apple.os.update-...⁩ 14.9 GB    disk1s5s1

error: container /dev/disk1 is mounted. repairs in a mounted container is not yet supported.

Same error message— error: container /dev/disk1 is mounted. repairs in a mounted container is not yet supported.

Please could you tell me the command for that?

OK, thanks, I'll do that now.

By the way, this is what I would like to try next:

"I was trying to install Boot Camp using macOS Big Sur. For some reason, Beta 1 installed some Snapshots on the Macintosh HD volume that prevented the disk from being partitioned. I did eventually get it working, but the solution did not come from me. Credit goes to user tomadimitrie on the Mac Rumors forums who found the solution.

• First I booted into macOS recovery mode, and made sure that Macintosh HD was mounted in Disk Utility.
• Then I opened Terminal from Utilities > Terminal.
• Then I needed to find out the id of the Macintosh HD drive by using the command diskutil list, and finding the IDENTIFIER of the Volume called Macintosh HD. This ID may be different in recovery mode than the one in regular macOS, so make sure you run the command in the recovery mode terminal. The IDENTIFIER looks something like this: disk1s1.
• Then run the command diskutil apfs listSnapshots <Macintosh HD's disk id> to list all the snapshots, all which start with com.apple.os.update-.
• Then delete each Snapshot by using the command diskutil apfs deleteSnapshot <disk id> -uuid <snapshot uuid from the previous command>. The last one should return an insufficient permissions, but that is fine. Reboot into macOS."

But I need to know if would be safe to delete that snapshot, seeing as (after running First Aid and before trying to partition with Boot Camp again) Disk Utility shows this: