error: snap_metadata_val object (): invalid extentref_tree_oid (0x0)

I have the same error as you. In my case it turned out to be a dateless time machine local snapshot.

To check for it, open a Terminal and list local time machine snapshot by running the
command "tmutil listlocalsnapshots /"

It will show entries like "com.apple.TimeMachine.YYYY-MM-DD-HHMMSS"

If you see dateless snapshot, delete it use "sudo tmutil deletelocalsnapshots YYYY-MM-DD-HHMMSS"
with the actual date of the dateless entry. You will need to enter your user password.

# tmutil listlocalsnapshots /

com.apple.TimeMachine.2017-10-04-014927 (dataless)

com.apple.TimeMachine.2017-10-04-024750

com.apple.TimeMachine.2017-10-04-024752

com.apple.TimeMachine.2017-10-04-024835

com.apple.TimeMachine.2017-10-04-024852

# sudo tmutil deletelocalsnapshots 2017-10-04-014927

Password:

Deleted local snapshot '2017-10-04-014927'

# tmutil listlocalsnapshots /

com.apple.TimeMachine.2017-10-04-024750

com.apple.TimeMachine.2017-10-04-024752

com.apple.TimeMachine.2017-10-04-024835

com.apple.TimeMachine.2017-10-04-024852

@LittleLebowski

same here on High Sierra 10.13.3

fixed with

tmutil deletelocalsnapshots 2017-11-27-194607

where you can verify the date is NOT listed in

tmutil listlocalsnapshotdates /

P.S: sorry, didn't see that there were more answers on page 2

Do not copy the full output, only the timestamp portion when you run the deletelocalsnapshots.

Assume the output is com.apple.TimeMachine.2018-03-25-021312 (dataless)

sudo tmutil deletelocalsnapshots 2018-03-25-021312

I had this same problem, preventing me from installing the 10.13.1 OS update (which failed silently whilst looking like it had worked, grrrr).

I rebooted to recovery, applied disk first aid and saw the error in the title of this thread, which is how I found this thread.

Followed the instructions here and lo and behold, removing a dataless local time machine backup solved the problem.

I regularly switch my laptop between GMT and GMT+1. I wonder whether there’s any commonality there? I also back up to two separate time machines in the different time zones, so at any one time I always have one time machine backup failing whilst the other succeeds.

use following command :

sudo tmutil deletelocalsnapshots 2017-11-27-194607

They are very different.

localsnapshot is file system snapshots supported by APFS volumes.

it's not a actual physical path where snapshot is stored (cloned change).

see tmutil man page:

delete path ...

Delete one or more snapshots, machine directories, or backup

stores. This verb can delete items from backups that were not

made by, or are not claimed by, the current machine. Requires

root privileges.

deletelocalsnapshots date

Delete all local Time Machine snapshots for the specified date

(formatted YYYY-MM-DD-HHMMSS).

Great. Thanks for this, I found a dataless snapshot, deleted it and DiskUtility now runs clean.

Thank you

You should write this command:

sudo tmutil delete 2017-11-27-194607

Yep. - there was a dataless snapshot on my SSD that was causing disk verification to fail exactly as described. Removing is allowed the verification to complete. This with High Sierra 10.13.4.

I suggest you run disk utility on the volume first before this.

see Verify a disk using Disk Utility on Mac - Apple Support

If there is huge amount of disk space used for localsnapshots, you can try thinlocalsnapshots verb of the tmutil.

TMUTIL(8) BSD System Manager's Manual TMUTIL(8)

NAME

tmutil -- Time Machine utility

...

thinlocalsnapshots mount_point [purge_amount] [urgency]

Thin local Time Machine snapshots for the specified volume.

When purge_amount and urgency are specified, tmutil will attempt (with urgency level 1-4) to reclaim purge_amount in bytes by thinning snapshots.

If urgency is not specified, the default urgency will be used.

Google thinlocalsnapshots for suggestion.

I have found Disk Utility in 10.13.5 checks each snapshot in a way I haven't seen before. Fixed my errors automatically. Has Disk Utility been updated?

If you look at the suggested fixes, none include "com.apple.TimeMachine." in the tmutil delete command.

It worked for me when I omitted it, but tested with it included and got a similar error.

Can you try run all commands as super user (root). I know the document does not states that.

Prepend all those tmutil commands with sudo, e.g.

sudo

Sorry, I'm out of idea. I suggest you talk to apple support directly.

The only thing I could think of is to restore the Mac from Time Machine backup.

About macOS Recovery - Apple Support