Hi,
I found a post that is very similar with yours and he solved it by changing driver letter.
For more information, please visit:
USB Storage Device Does Not Appear In My Computer
http://help.lockergnome.com/windows/USB-Storage-Device-Computer-ftopict519043.html
Also, please check if the following registry value is 1. If not, please change it to 1.
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced\Separa
teProcess
Please try the steps listed in this post. If this symbol still continue, please post here again.