Vereesh,
I've also experienced this and I'm pretty sure that the cause is as Aux says.
My guess is Drive H: maybe? If you select the network drive that's most likely to be causing the conflict, right-click and choose disconnect & then try connecting the USB drive, it should connect.
You could either change the network drive mappings or you may be able to configure your USB Pen Drive to connect under a different drive letter.
Jon