A person answers.

Write to support@diskist.com and a person answers. Say what you did, what you expected, and what Diskist did instead.

A lot of the disk is hidden space

macOS keeps some folders private, and Diskist counts what it cannot read as hidden space. To measure more of the disk:

1. Open Diskist's Settings, then Access.
2. Click Open System Settings.
3. Turn on the switch beside Diskist. If it is not in the list, click + and choose it.
4. Quit Diskist, open it, and scan again.

Snapshots and the other volumes of a disk stay hidden space. No app from the App Store can open them.

macOS asks about Desktop, Documents or Downloads

The first scan that reaches each of those folders makes macOS ask, once each. Choose Allow to count them.

The space did not come back

Move to Trash puts the collected items in the Trash, and the space returns when you empty it. To skip the Trash, open Settings, then General, and choose to delete at once. Deleting at once cannot be undone.

Something I removed

If it went to the Trash, open the Trash in the Dock and drag it back. The Cleanup Log, ⌘L, lists every item you removed and where it lived.

An item will not go in the Collector

Diskist does not offer to remove macOS or the folders a Mac needs: the system folders, your home folder, and the standard folders in it, like Documents and Library. What is inside them can be collected.

Removing Diskist

Drag Diskist from Applications to the Trash. Its settings and the Cleanup Log stay in its folder under ~/Library/Containers; delete that folder too if you want them gone.