When a USB drive is recognized, but if you are not able to read from it or write to it, there might be some hardware problems. Here is the to do list for tackling the problem
1. If your USB device is not recognized, remove it and put it back. If it is still not recognized, either there is a fault with the USB or the Computer or you do not have such permissions for inserting removable media. Or the drivers are not working properly. Try installing the USB drivers.
2. If the USB drive (pen drive or any memory card) is detected, but if you are not able to read from it or write to it, then you will have to remove any extending wires and try inserting the pen drive in to the USB port directly.