[lpc2148] usb mass storage can't be formatted on windows OS

I build a code, which is based on the USB Memory Device demonstration for the Keil MCB2140 Board with Philips LPC2148.

any functions work fine.but the format action on windows OS.
when I attach the device on PC, right click the u disk, and select the format menu, the start format action on the following window, but nothing happen,I add some trace in the code at write & read callback function in mscuser.c, but nothing is printed.
Note, on the format window, I don't check the "fast format" checkbox.
if I check it,then it works fine.

please someone give me some advice. how does windows works on format action ?

thanks.