I have an Iomega eGo 300GB USB external hard drive. I have installed Lubuntu to a partition on the drive.

When I tried to reboot from the external drive I get the message
file system not recognised
grub rescue>
I went into Xubuntu on the internal hard drive, mounted the external drive and entered
sudo update-grub
- it showed that it had found the Lubuntu installation on the external drive
then I entered
sudo grub-install /dev/sdc
(sdc is correct for the external drive)

When I tried to reboot from the external drive, the system ignored it and booted straight from Xubuntu on the internal drive - with no boot menu.

I have successfully installed Lubuntu on a USB Flash drive before.

Can anyone help please