I've had a LaCie 500GB D2 external HD running on an XP machine for about a year. A couple of days ago, it stopped being recognized. I tried it on several others computers (all XP) and had the same problem. I connected it to a Windows 7 machine and it was recognized but I couldn't access it. I formatted the drive using Windows 7 and it works fine on that computer but still won't work on any one running XP. When I plug it in, I get no indication that the device is detected. It is not listed in the device manager or the disk management utility. I have replaced the cable.
Problem solved. When I took the drive home to test it, I used a spare AC adapter. that's why it worked on my home (Win 7) system. It turns out that the adapter was the problem. My guess is that one of the voltages was low. I replaced the adapter and the drive is working fine.