Linux handles it's own fragmentation, but you get huge left-over files down a few directories, behind hidden files and directories. I was amazed at how my main disk (solid state) was clogged up.
So, get K4dirstat from the main repositories. You actually have to open a directory or a disk. That stumped me for a long time, since there are no manuals. Duh!
When it stops reading, it gives a really nice picture of the files on the disk.
It's so cute and you can click on each rectangle, and it shows you what the file is. Then, if it something you don't want, you can delete it. Neat!