Comment 3 for bug 523903

I think I'm running into the same problem. I've installed several kernels, and then removed some of them using dpkg --purge.

Now, update-initramfs -u complains:

$ sudo update-initramfs -u
update-initramfs: Generating /boot/initrd.img-2.6.33u1
Cannot find /lib/modules/2.6.33u1
update-initramfs: failed for /boot/initrd.img-2.6.33u1

It seems like this Debian bug might be related, although it is quite old,
and reportedly fixed 2 years ago on their system:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=420245

Cleaning up the contents of /var/lib/initramfs-tools/ (to make
it consistent with the kernels installed in /boot) fixes the
problem.