This post helps you to format unformattable pendrives.your pendrive becomes unformattable due to some virus attacks.this Trick helps you to format even unformattable pendrives(also memory cards).This trick is done with the help of command prompt.
Follow the given steps to do that

(backup important data in the device first)
(follow the only steps as given)
(plug in usb before doing this)


start>run>cmd

then type:

Format/x J:

(J refers to usb disk drive letter it may be different in your case so replace the letter)
then press enter

you are done !!!
 SOURCE : INTERNET
get more command prompt tricks