Firewire Hard Drive is not being detected

How about SMC Reset or PRAM reset?
Apple Hardware Test?
Unplug everything for 15 minutes, power cord, cables, FW power adapter.

Did you set the drive to Master when you installed it? and did you eject or unmount the drive the last time it was used? Seeing you can use USB2, did you see if Disk Utility and Disk Warrior found any problem?

Tried swapping the the cables within the enclosure?

I cannot get my Mac Pro to see my external hard drive
(Mercury Elite Pro) when it connected by Firewire 400
or 800. I tried both front and back ports, neither
work. When connected by USB cable, the drive DOES
work so I know it's not a problem with the drive.

Using the Disk Utility, the drive does not show up at
all. I've checked all the usual suspects (cable is
fine, the drive is on, etc.). BTW I was able to use a
Firewire drive with this system a few months ago so I
know it did work at one point.

Any ideas on what I should try? Thanks.

I recently had the same issue with Mac Pro 2.66. Had to have Apple Care replace the Firewire Board. Worked with USB, but not FW 400-800. The repair center didn't have an answer, but mentioned these things do happen. Do you know when the last time your machine was updated? There have been other posts with concerns about Updates and conflicts with Firewire Firmware.

Most people with trouble had trouble to begin with. However there was a serious problem with 10.3.0 that would disable a FW drive if it was connected while installing and updating the system. That lead to a "Never allow your FW drives to be connected and on during an update." And there may have been another OS update that affected FW drive, though not sure if it could be called firmware change or update.

All of the Oxford based FW cases should be fine from what I have seen, and it is non-Oxford (WD, Maxtor etc) that run into trouble, or older models.

There were posts related to issues with firewire and 10.4.8. I believe that the problem occured with having the drives attached during the update, and fritzed the ports as their firmware was being updated. There is also issues posted with Lacie's version of their own Firmware updater for their external HD. This is what occured to me, and what caused the blown board. Would this apply to you at all? The ports on the back work, as well as the monitor is that correct?

Cheers,

It isn't maybe the cables, but any current they hold that affects LaCie drives. Which is why they need to be totally unplugged.