Your Time Machine backup disk might not always be available, so Time Machine also stores some of its backups on your built-in startup drive and other local drives. These backups are called local snapshots. One snapshot is saved every hour. If you’re using macOS High Sierra, another snapshot is saved before installing any macOS update.
Snapshots older than 24 hours are automatically deleted according to Apple, but I’ve seen snapshots many days old. And to make sure that you have storage space when you need it, snapshots are stored only on disks that have plenty of free space. When storage space gets low, additional snapshots are deleted, starting with the oldest. That’s why Finder and Get Info windows don’t include local snapshots in their calculations of the storage space available on a disk.
I think the concept is ok, however I don’t want / need it. There is currently no option to turn this ‘feature’ off. Also, I think it makes people think they don’t have enough disk space when looking at the disk utility app while they actually do have enough space available – another reason to upgrade to a new Mac.
The solution that worked for me is to run the following command in terminal:
tmutil listlocalsnapshotdates / |grep 20|while read f; do tmutil deletelocalsnapshots $f; done
The above command removes all the local snapshots. Warning, this will mess with the Timemachine Backups clearly – do it at your own risk.
More information about local snapshots here: https://support.apple.com/en-ca/HT204015