What model Lacie did you pick up? When you were getting 55mb/s how much of the lacie's space was full?
If its a Lacie Rikiki its a 5400rpm hitachi hdd in it (or at least that is what is in my 1tb rikiki) thus 55 is not outside the realm of possibility!
When in doubt...upgrade the NEC controller firmware and drivers. Your mobo mfger's driver and firmware are probably atleast one revision out of date (or more). I get mine from
SONY/NEC/RENESAS Drivers & Firmwares