You must format it.
1. Click Start, click Run, type compmgmt.msc, and then click OK.
2. In the console tree, click Disk Management. The Disk Management window will appear.
3. On this disk management window, you can see some drives including the portable drive. Right click on your portable drive and click change drive letter and paths.
4. Now a assign a new drive letter which is not being used by any of your drives in My Computer.
5. After assigning the new drive letter your portable drive will show up in my computer with other drives
We hope the above solution will help to fix this trouble.
It's available for download on link below:
Plug your pendrive into your computer and let it detect it and format after detection