The harddisk manufacturers, and it seems palimpsest too, are counting in decimal system, df is counting in binary. So one kilobyte for the manufacturer is 1000 Bytes, but for the computer it is 1024 Bytes. The difference adds up to 41 GB in your case.
Last edited by TobiSGD; 11-18-2010 at 12:36 PM.
Reason: fixed typos and grammar