The folder you're seeing has nothing to do with the network volume you created; it just happens to have the same name. This is a new feature in Lion which keeps track of file changes on laptops when you're away from home so that you still get the benefit of file versioning that Time Machine provides even when your external backup drive is not connected.
You can find more information about this feature in John Siracusa's Lion review at Ars: http://arstechnica.com/apple/2011/07/mac-os-x-10-7/18/
All signs point to you not having 400 GB (or even 26 GB of space used from .MobileBackups) and with the repeated edits, I'm not even sure we're on the same page, but for others - here are some things to try:
Rather than re-enabling things, try giving the system 5 minutes to clean up before the restart
sudo tmutil disablelocal
sudo shutdown -r +5
Then when the machine restarts, look for that folder and delete things using Finder or terminal from /.MobileBackups once the machine is restarted with the local store disabled.
First check for a huge file in / with:
sudo ls -sa1 / | sort -n | tail -4
We know the large files aren't anywhere else but Applications and Users - but you could use this answer to search for smaller files (say 1 GB or larger)
If you're looking to just know what the size of various folders is - this long running command (14 minutes on an SSD MacBook Pro with 200 GB total allocation / 6 minutes on Mac Pro with about the same) will show you a list of the largest folders - hidden or not.
sudo du -smx /* /.* | sort -n
If that folder no longer shows using that space, your next recourse is to see if the filesystem catalog and free space accounting can be fixed. That would be the last step before backing up, erasing and reinstalling the OS.
Boot to single user mode: Hold Command + S when the boot chime is sounded and hold it until white text on a black screen appears.
/sbin/fsck -fy /
repeat the above check+repair until there are no errors and then
halt
Best Answer
Mojave stores Time Machine snapshots as APFS snapshots.
You can list these snapshots using
tmutil
:These snapshots are not exposed anywhere in the filesystem by default, but they can be mounted manually using
mount_apfs
:Make sure to unmount the snapshot when you are done with it: