Lacie 320gb Ext Hd - Not Mounting

I bought an imac, 2.4 Ghz, 2GB ram, a week ago. i also bought a 320 Gb lacie ext Hd, for my imac. i had hooked it up using the firewire port and everything was working fine. Now, all of a sudden the disk does not show up on the desktop..
I've tried the following:

1) Turned off HD while connected, and then turned it on.. Did not show up on desktop, nor on disk utility. I did this using both the firewire connections on the hd and on the imac.

2) I then tried it with the USB 2.0 and it popped up in a second.. and seems to be working just fine.

My question is..
1) Is there something wrong with the firewire port on the imac or ext HDD..? I have a 2 yr warranty on the drive, and i can get it replaced, but what about the mac, how do i check if the firewire port is ok? I read a couple of threads on the forums, and saw that others have the same prob too, but no one seemed to mention if they ever ended up getting the firewire connection working again..... or is there something else i am missing out on altogether..?

2) Is USB 2.0 faster to transfer data or the firewire cable?

I would check the System Profiler in Utilities. (applications/utilities)
It would show if FireWire is ok. Could also use the Apple Hardware test. ( hold D during startup for intel based)
Can also check the logs with the Console app. (utilities)
Wouldn't hurt to get the LaCie update tool off their website as well.

i tried system profiler and the firewire port seems to be ok.
the hd still dont work though...

Have any other hardware or cables to try?
I believe the specs for the iMac have 2 ports. 1 400 and 1 800. Have you tried both?
Did you check the other ideas I posted?

tries both the 400 & 800 cables... no luck.
works instantly on usb 2.0 though...

by the way is usb 2,0 faster or firewire..
i tried transferring 2 gb using usb 2.0 and it worked beautifully, couldnt tell the diff....

Testing has shown FireWire to be faster. Writing speeds at almost 50% faster.