I connected a live USB stick on startup and this happened:
welcome to GRUB
->error: filesystem unknown ->Entering rescue mode -> ls ->(hd0) (hd1) (hd2) -> ls (hd0,msdos0) -error: no such partition -(hd0): Filesystem unknown -> ls (hd0) -(hd0): Filesystem unknown -NB 1.I run windows7(64bit) 2.i didn't have dual boot 3.If I dont connect the live usb my PC starts on windows7 just fine.