Shrinking snapshots

Sometimes snapshots turn out to be larger than expected and free space shown in the Finder falls worryingly. Although this should be managed automatically, it can cause alarm and block installations and updates. The Storage view of About This Mac may not be helpful, as this gets lumped into System Data, and only Disk Utility gives an accurate picture.

While we save space in backups by excluding large files like virtual machines, a snapshot copies the file system for the whole volume, including everything we don’t want backed up. It also grows larger the longer you leave it, so forgetting to hook up your notebook’s backup disk for a couple of days can soon steal tens of gigabytes. Moving those files to a separate excluded volume spares you that overhead, and in APFS comes almost cost-free.