Hello, try this...
reset FW bus, same goes for USB reset...
Reset the Firewire bus
If your Firewire or USB isn't recognizing any device. A solution which has worked for some whose hard drive became invisible in 10.4 was simply to follow these four steps to reset the Firewire/USB bus:
1. Shut the machine down.
2. UNPLUG the power lead to the computer and any firewire/USB device.
3. leave it for 10 minutes.
4. Connect back up and reboot.
You mentioned "Mac Pro. Do you have a PowerMac? The instructions BDAqua gave do work.. but sometimes you need to go a bit deeper. Try a PMU reset.. or if you have the Mac Pro(non-powerpc mac), do an SMC reset by shutting off the mac pro, pulling the power cord out and waiting 5-10 seconds and plugging it back in.
If the problem keeps persisting, try contacting Apple or make an appointment with the Genius Bar. I would say a PMU/SMC reset would do the trick.
USB and FireWire Quick Assist
Common fixes for USB device issues (unrecognized, etc.)