GRUB error: failure reading sector

[Haines Brown <haines@histomat.net>]

I am running Sid on a Thinkpad x250, and decided to create a rescue USB
key by using grub-rescue-pc package.

On a 1 Gb unmounted Ex3 key, which is /dev/sdb, I do

  # cp /usr/lib/grub-rescue-pc/grub-rescue-usb.img /dev/sdb 
  this takes a while.

When I boot to key, I get "GRUB" in upper left corner and a long wait
during which the key LED flashes. After a while I get:

  GRUB error: failure reading sector 0x48c from 'hd0'
  Entering rescue mode
  grub rescue>

I suspect hd0 refers to the key and GRUB can't find its own grub
folder. I don't know how to proceed.