Often the reason that the computer doesn't see USB drive, is faulty connectors. Make sure the indicator light on the drive flashes. This is a signal that passes through the connector an electrical current. In the absence of a light signal to move the drive to another port, if any. If the computer still does not see the USB drive, the problem most likely lies in the connector of the stick.
If the drive indicator light comes on, but the computers it does not recognize, most likely the problem is faulty USB drive. To extract information from such a stick, you should see a specialist.
If the computer shows the message "USB Device not recognized" is probably the system assigns the drive letter that is already occupied by another disk. To resolve this problem, open "control Panel" and go to "administrative tools", select "computer Management". Next, open the "disk Management". In the list of all your drives find the USB flash drive. Click this line, right-click and select "Change drive letter". Specify an available drive letter in the system and click OK.
Sometimes the computer does not have a special driver so it cannot read the drive. This conjecture can be checked, if you go through "control Panel" in "device Manager". In the list that appears you should find the item "USB Controllers". In the absence of the USB drivers, the disk will be highlighted in yellow. Drivers need to be downloaded from the website of the manufacturer of the flash drive and install it on your computer.
The viruses in the operating system can cause a variety of problems in the computer. Sometimes they interfere with proper operation of the USB disk. In this case, you need to scan the hard drives antivirus software and remove viruses.