I've dealt with this kid of thing before. There are a couple things to check. First of all, i assume it works in other computers, if you have not done this then please verify. I will also assume that since you did not state it, that you are working with windows XP There are a couple places in the process where it can break down. The easiest way is to start with the most common.
Right click on My Computer, and click manage. On the left side there should be Device Manager. Click on that and the right side will populate with a device tree, look for anything with a yellow or red icon, if nothing, the proceed with the next step.
On the left side of the same window, there is Disk Management. Click on that and the right side will populate with a graphical representation of you hard disks and removeable storage. Look to see if your drive is listed there. If it is, look at the drive letter, and if there is a duplicate of the same letter (Sometimes mapped drives can have the same letter as a physical drive, this is an error and should not be possible.) Right click on your drive and click change drive letter or path. Select an available letter and it should work.
Let me know where the things don't show as i said and i can help you from there.