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Try right clicking computer, select manage and in the left column select Disk Management. Look in the lower part of the screen and see if you see it, it will not have a drive letter. If you see it with no drive letter, right click it, select 'change drive letter or path', next screen select Add and the next screen choose a drive letter from the drop down list. After you have assigned it a drive letter, it will show up in explorer. If there is data on the drive you want to keep and disk management wants to initialize or format it, don't do it as it will erase everything on the drive. If there is no data that you want to keep on there, go ahead and create a simple volume and format it.
