when I plug my USB flash in my Pc it asks me to write the administrator user name and password.. I wonder why??

That can be caused by using a U3 pen drive which creates two drive letters in Windows Explorer when it's plugged in. Try using a non-U3 regular pen drive instead.
yes I tried ,, besides this happen with all usb flashes I use ??
could it have reason of user rights on the network ???
