Ok so I originally formatted the drive for leopard and yesterday I was working on a project on an Tiger computer. Everything was fine, I worked all day and left. I came back the next day to work on a different tiger computer, I was transferring files and then all of a sudden, I get this error message: "Driver not found". Then I notice the transfer is frozen, I cancel it out and then reboot the drive. At first, it wouldn't connect to the volume (Sean1) but I could see the drive in disk utility. I tried a repair but it didn't work. Then I jumped to another tiger computer and now the drive won't even show up in disk utility. Help!

I think you saw the drive as it was dying. It hadn't completely crapped out, so you could see it in the disk utility. It breathed it's last breath before you moved it to the second computer and that's why it doesn't show up at all. You could continue to try it in various computers, but indications are that would not be helpful.