External firewire HD not working properly

I have bought a LaCie external HD with Firewire and a connection cable 6pin-4pin (which is not included in LaCie delivery) for use with my LapTops.
I have a Dell Inspiron 8100 with W2000 SP4 and an Acer Travelmate 630 with XP pro SP2.
I have formatted the HD NTFS on the Inspiron and for two days all worked fine, including connecting the unit on one computer and after on the other computer and so on.
Suddenly, restarting the Acer with the HD connected has given as result starting the procedure saying that the unit must be checked and after a sequence of lines saying the sector number xx cannot be read. Redoing the procedure may not give the same result: it may happen that Windows starts normally, but more slowly than normal.
Hot docking the unit now gives on both computers erratic results, with one common characteristics: the computer slows down and somtimes IE is freezed. Sometimes the unit is not recognised, sometimes it is after much mumble mumble, and after opening some directories the system claims that the unit is not formatted and if I want to format it. In another occasion I have tried to start formatting myself, and I got the answer that there are no sufficient information to do so.
The unit always is displayed correctly in the Device Manager, but not always in the Resource Explorer.
I have brought the unit to the shop wher I bought: they could not help me because with their desktop computers the unit worked fine.
I have passed several hours in reading posts on this forum and other sources: I have found very few similar situations, speaking about the updated SP or the connection cable. I suspect indeed that one possible source of problem could be the connection cable, which is not original from the HD manufacturer, and their user guide claims that only their cables may be used. I am also waiting for answers from LaCie technical support.
Can anyone help me having had similar adventure?
Thank you

Problem solved. It was with the cable. Using the original 6p-6p cable (1/2 meter long) with a reduction from 6p to 4p, in place of the cable 6p-4p (1 1/2 meter long) purchased separately has fixed the issue