Originally Posted by goumba
the Debian team has taken care of the chroot step for you. Boot your Debian installer CD/flash drive and select Rescue mode from the menu. It will then allow you to choose a device as the root device. Do so, and it will drop you into a chrooted environment.
I have done this and it has worked. Thank you very much!
The debian installer indeed has a rescue mode.
On the rescue mode it actually offers an option to install GRUB onto a device. Thus chroot is not required.
Unfortunately, it offered to install to sdb or sdg and I couldn't tell which usb was the target OS and which was the debian installer I was using. Since sdg had 2 partitions and sdb had only one - I guessed (rightly) sdb was the target OS.
On reboot, the usb loaded debian. So GRUB was successfully installed onto the MBR of the usb.
Clearly then, if I use debian installer to install an OS onto usb, then GRUB will fail to install onto the MBR of the device.
But I can then use 'rescue mode' on debian installer and install GRUB again onto the usb - and this time it will work.
Thank you again for your help!