This sounds like it might be a case of the drive losing its formatting. If you have windows visa or 7, go to the search box in the start menu, type "disk management", and select the tool called "create and format hard disk partitions". In this tool, you will be able to see all the different storage devices connected to your computer, and the format of each of them. See if the drive is listed at all, and if so, if it has a format associated with it or if it's all just 'free space'.
Answer: Click on "Start" and search for "Disk Management" and then delete the partition that does not have a letter. Now create a new partition and choose the letter you want. That's all. Your hard drive should now be ready.
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