Unremovable red- no access / circle folders with old external hard disks names

I have red circle folders in my Macintosh HD root folder.

They have the names of old external hard disks that I don't use anymore (!) and have no files in.

I would like to remove them but even if I am administrator on my Mac, when I try to remove them either via finder or via terminal, the computer says I have not enough privileges...

And I get the same answer if I try to change privileges on any of them via finder.

How can I remove those useless folders?

Thanks

Nicola