Sometimes when you plug in an external hard drive, thumbdrive, or flashcard, it may not show up because it is trying to use a drive letter already in use. To fix this you will have to manually change the drive letter it is using.
Right click on "My computer" and select "manage". Go to "disk management" and see if the physical drive is visible and what letter it is using. If it is conflicting with another drive, then right click on the drive and change it's assigned letter.
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