USB thumb drives (also known as USB sticks, jump drives and flash drives) are used to store data and transfer files from one computer to another. Taking thumb drives from one computer to another can cause drive letter conflicts that prevent them from being recognized as an available storage device. This problem occurs when thumb drives are assigned drive letters that are already being used by the computer they are being plugged into. The problem usually occurs when you take the thumb drive from a laptop computer and plug it into a networked workstation. Use the following steps to make your device visible again.
Navigate to the "Start" button and enter "diskmgmt.msc" (without quotes) in the Search box and press "Enter." The Disk Management dialog appears, displaying all available drive letters.
Right-click on the drive letter that corresponds to the USB drive. Select "Change Drive Letter and Paths" from the pop-up menu.
Click the "Change" button to display the Change Drive Letter or Path window. The USB device's current drive letter will be displayed in a drop-down list. Select from the drop-down list a drive letter such as X, Y or Z that will be less likely to conflict with existing drive letters. Press the "OK" button to save the change.
Insert the USB device into the computer and confirm that the device appears on the list of drives with the new drive letter.