Disk too full

[Konrad Neuwirth]

Dear reader, 

there is probably something very, very obvious that I am missing, but one server of ours is claiming so much more disk space is filled than current use warrants and I don't see how I can free that again. 

It's a drive that contains five pfs; four master and one slave. 

I've reblocked the pfs, but still there's too much overhead. 

How can I investigate where the blocks went, and particularly: How can I free them? 

Thank you,
 Konrad