Why wont my backup drive appear on desktop?

I have a Mac Mini 1.83 Ghz running 10.6.4 that I backup to a 1TB Newer Technologies dedicated drive that will not appear on desktop or finder yet occasionally it backs up the system but not always as scheduled. At first I thought it was a drive issue so the retailer replaced the 1TB drive only with the same results. I have it connected via USB.
Any ideas?
Thanks
Mike

btbrossard said:

Does it show up in Disk Utility? _ Yes!

What format is said drive?

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Mac Os Extended Journaled

DewGuy1999 said:

Is your internal hard drive showing on the Desktop?

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Yes the internal drive AND another 300 gig Newer Tech Mini stack appears.
The other is also connected via USB.

DewGuy1999 said:

I think what has happened here is that the hard drive's visibility bit is set to invisible. This AppleSprite: Controlling Hard Drive Visibility With Onyx article tells how to use OnyX to change a currently visible hard drive to invisible. Follow the instructions in that article but instead of making it invisible, make it visible and your drive should show up.

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This opened up a whole bag of worms. It displayed faults on my Boot drive!

DewGuy1999 said:

I don't understand. Changing the visibility of your external hard drive doesn't have anything to do with your boot drive.

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No, when I ran the software it found faults on my boot drive that I need to correct first. I had to locate the OS discs first.