Dear Professionals !

There are 2 Win8 partitions on my USB-HDD - say F0,3 Gb) and G255 Gb). All works and Win8 loads.
I've installed Ubuntu 11.04 to the next 2 partitions on this USB-HDD - say sdb5(252 Gb) and sdb6(3 Gb - swap).
Before installation process Ubuntu asks - where to install GRUB and I point to the whole USB-HDD(sdb).
After Ubuntu installation the next Windows 8 loading is impossible.
Ubuntu doesn't load - when I try to load from this USB-HDD I see the empty screen.

1) What have I to show to Ubuntu - where to install GRUB ?
2) How to restore Win8 booting from those F: and G: ?