Lacie d2 external hard drive malfunction?

I've got an Gigabyte 7NNXP mobo with an Athlon 2100+ processor, 1GB RAM, running XP. Everything running smoothly, except that my Lacie d2 160GB external hard drive, which I use for backup, seems to be malfunctioning. I got a message yesterday when I tried to access it, saying that it wasn't formatted. Rebooted, and same thing happened. I use it only for backup, so I reformatted it and everything seemed fine. This morning I backed up all my files on the external drive using XP's backup program. No problems, and it backed up fine, but when I tried copying another file to the external drive, I got the message saying it wasn't formatted again.

When I unplug it and plug it in again it isn't recognised by 'My computer', but the 'Safely remove hardware' icon registers it as being hooked up to the system, and will let me unplug it after stopping it.

Has anyone had any similar problems, and if so, is the hard-drive malfunctioning or the computer itself?
Thanks for any help anyone can give.