Hi, I'm installing bt4 in a usb thumb and when I arrive at the part which I need to 'install' the grub, using: "grub-install --no-floppy --root-directory=/mnt/sdb1 /dev/sdb" it returns me the following error:

grub-probe: error: Cannot find a GRUB drive for /dev/sdb1. Check your device.map.

[: 494: =: unexpected operator
The file /mnt/sdb1/boot/grub/stage1 not read correctly.

How could I take care of this ?
Thanks for the help