LaCie Rugged 500GB is incredibly slow?

I have a LaCie Rugged 500GB FireWire and a 2007/2008 24" iMac and am trying to transfer about 250GB of data (a single file) from the Rugged to the iMac. The problem is when the transfer gets above 200MB it slows down to about 1MB every 3-4 seconds which is ridiculous. This happens regardless of what connection I use. I also have a WD My Book Studio and have no problems with speed on that. HELP!