Granting access
How to let DiskPeek see the folders you care about.
Why you have to choose the folder yourself
DiskPeek is sandboxed, which is what the Mac App Store requires. No app in the store can be given free run of your disk. The only way it can read a location is if you select that location yourself in a system panel, which hands the app permission for that folder and everything inside it.
That is why clicking a volume shows a folder chooser rather than just starting.
It only asks once
When you grant a location, DiskPeek saves a security-scoped bookmark for it. Next launch it still has access, so you are not asked again. Granting a whole volume covers every folder inside it too.
Adding a restricted folder later
Some folders stay locked even inside a volume you have granted, because macOS protects them separately. Select one in the browser and choose Grant Access, pick that exact folder in the panel, then use Recheck Inaccessible Folders to fold it into the totals.