Comment 0 for bug 1682160

While not common, and probably not desirable, if /etc/fstab is not present, update-grub-legacy-ec2 will fail. Thus, if you don't have an /etc/fstab, an install of a kernel package will run /etc/kernel/postinst.d/x-grub-legacy-ec2 and thus fail install.

recreate:
% sudo mv /etc/fstab /etc/fstab.orig

% sudo dpkg-reconfigure grub-legacy-ec2
Searching for GRUB installation directory ... found: /boot/grub
Cannot determine root device. Assuming /dev/hda1
This error is probably caused by an invalid /etc/fstab
awk: fatal: cannot open file `/etc/fstab' for reading (No such file or directory)

ProblemType: Bug
DistroRelease: Ubuntu 17.04
Package: grub-legacy-ec2 0.7.9-90-g61eb03fe-0ubuntu1
ProcVersionSignature: Ubuntu 4.9.0-15.16-generic 4.9.5
Uname: Linux 4.9.0-15-generic x86_64
ApportVersion: 2.20.4-0ubuntu4
Architecture: amd64
Date: Wed Apr 12 15:05:56 2017
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=C.UTF-8
SourcePackage: cloud-init
UpgradeStatus: No upgrade log present (probably fresh install)