Archive won't unzip

Trying to open a .zip file on my LaCie, I get the message:

Archive 'RENFIELD.zip' can't be unwrapped in 'Bapeldee Hard'.
(fault 1 - process not authorized)
translated from dutch: Archief 'RENFIELD.zip' kan niet worden uitgepakt in 'Bapeldee Hard'
(fout 1 - Bewerking niet toegestaan)
'Bapeldee Hard' is the LaCie.
I've tried copying the file to my iMac Intel, with no result.

What should I do to open the file.

iMac Intel Mac OS X (10.4.8) LaCie 300Gb external harddisk