I have a laptop with Windows 8 installed and an external hard disk with Ubuntu 13.04 installed which I connect through USB. I have installed grub on my external hard drive. Previously I was able to boot from my external drive without any problem. But now when I boot, my external drive is not displayed in Boot Options menu. I have one ntfs partition on my external drive and windows detects it. So, my laptop or external drive does not have any hardware problem.
I tried to boot from my external hard drive on another computer which does not have UEFI or Windows 8. It has both Windows 7 and Ubuntu 13.04 as dual boot. When I first booted from external drive, I got grub rescue screen. I followed the procedure given on How to fix "error: unknown filesystem. grub rescue>. I was able to boot into Ubuntu, but it was showing '/boot/efi' is not ready. I skipped mounting it and it worked. I thought the issue is fixed. But when I connected to my laptop, again it was not getting detected in Boot Options menu.
What could be the problem and what is the solution for this?