Importance of Backing Up Data

If any of you have ever lost vital data from computer glitches, you already know the importance of having backups. Well, I made the mistake of adding an external hard drive to my iMac. The problem was that I put all of my data onto the drive, then deleted it from my hard drive, because I was running out of space. However, now I have purchased an even larger external drive (250 gig), and I've got to get the data off the older drive, and onto the new one. Well, the old drive is labeled as unreadable. I have tried for the past 2 days to access the drive to no avail. It seems as though the last 6 months of design work is gone. I'm sick about it. I only have a week left to return the non-working drive, but I don't want to return before getting the data off of it. Does anyone have any suggestions?