DETAILS: I have an iMac G5 running OSX 10....

DETAILS: I have an iMac G5 running OSX 10.5.7. I purchased a G-Tech 913003-01 G-Drive 500GB Quad-Interface External Hard Drive in March and have been using it as a back-up drive ever since. Just this month I've been having issues with it not mounting. I'm using Time Machine to regularly back up my iMac hard-drive and another external G-Drive. It's connected via Firewire 400 but recently System Profiler shows the device as 'Unknown'. It won't mount now. It turns on and sounds like a normal startup. Even the light blinks, but it just doesn't mount and Time Machine doesn't recognize it.

Here are things I've tried.
1) Restart computer and restart drive. Still doesn't mount.
2) Changed Firewire 400 cable. Still doesn't mount.
3) Tried using USB. Still doesn't mount.
4) Tried Restarting Computer many times and turned drive on and off. Every now and then it mounts. But it's totally random and I can't seem to get it to mount at all now. Very rarely will it mount after I shut down my computer and drive, unhook everything and wait for 15 -20min.

FYI: I have another G-Drive (~160GB) which houses all of my media (music, images, movies). This drive I've had for 3years now and it never has any issues with mounting. It also uses a Firewire 400 cable.