Strange undestroyable folders

A friend of mine had a very very strange problem. Suddenly on his external hard disk there appeared a bunch of very strangely named folders like "%/sl94..." and other symbols which windows was not able to display. They occupied 300GB and clearly my friend wanted to delete them; unfortunately windows was not able to delete them. It gave always an error like "Path not found" or "Invalid path symbols".
I tried then to delete them with different data shredders but none of them was able to successfully delete them.
I have then run also different virus and spy-ware scanners over them but none of them found something.
I was stuck and at the end with my knowledge.

Then I found on the Internet the good old command line instruction:

del foldername

With that I was finally able to delete them all.