If you upgraded Grub 2 in the dual boot with both Windows and Ubuntu machine.When you reinstall Windows and then restore the grub 2,you may encounter “error 11: unrecognized device string” after select Ubuntu in grub menu.Following shows how to fix this error:

Select Chainload into GRUB 2 in this screen and press E


Select the line start with  “root” and press E in next screen.Then change the “root” into “uuid”(without quotes),press enter and then press B.You will back to grub menu,select Ubuntu and boot up.

Once login,open terminal and type:

sudo upgrade-from-grub-legacy