Ever since macOS Catalina was introduced, many Mac users noticed that their disks eventually get full without obvious reason. A quick scan in DaisyDisk typically reveals a big chunk of “hidden space”, which means the system is using more disk space than it’s possible to reveal by scanning, even with elevated permissions (scanning “as Administrator”).
The reason for this problem is that macOS is making temporary backups of the system, so called local snapshots, temporarily locking big amounts of disk space in an area of the disk that is fully opaque to the users. Eventually when the snapshots get transferred to the permanent storage, such as the Time Capsule, or as the snapshots become too old and get replaced by newer ones, macOS releases the disk space. However, the backup process is continuous and therefore there is always a certain amount of disk space that is locked by macOS for the snapshots.
This is how macOS Catalina works and it’s not supposed to even be a problem or require any maintenance from the user. However, in certain cases, you just need to free up the space urgently, and it’s quite confusing that you don’t have control over a big chunk of your disk.
Thankfully, the new version of DaisyDisk (4.10) is making it much easier to reveal the macOS’ hidden space, and for the part that cannot be revealed - understand its internal structure and even provide tools to reclaim it by demand.
In particular, the new DaisyDisk cuts down the amount of hidden space by revealing some obscure system items such as non-linked content of the “firmlinked” Data volume (under /System/Volumes/Data), the virtual memory volume (under /private/var/VM) and some other.
Besides, DaisyDisk now shows a breakdown of the hidden space that includes the following items:
Purgeable space. (You can also see it in Finder and Disk Utility when you inquire info for a disk). This is an amount of disk space, as calculated by macOS itself, which includes the snapshots too. You can forcedly purge this space by deleting it in DaisyDisk.
[NEW] Snapshots. This is a list of the temporary snapshots, with their estimated sizes. You can forcedly delete any or all of them in DaisyDisk, with a simple drag-and-drop, as you’d do with regular files.
Other volumes. This is the remainder of the system volumes that are used internally by macOS.
All in all, the new DaisyDisk is making a huge step to return you understanding and control of your disk on macOS Catalina, and probably provides the most exhaustive and informative view of your disk compared to any other disk utility.
This is a free update for the existing users of DaisyDisk 4.x.