The AS/400 has a very "easy-to-use" disk analysis tool.
GO DISKTASKS to see it.
That shows you a menu like this:
1. Collect disk space information
2. Print disk space information
10. Work with libraries
11. Work with folders
12. Work with objects by owner
The first step is to run selection 1 to collect disk information.
This starts a job that may run for 15 minutes or so.
When the job is done, use selection 2 to print reports from the disk info. The most interesting are:
1. print all libraries sorted from largest to smallest without file details
2. print details of any library to see each file and how much space it is using