At least [number]mb needed on the boot filesystem

So, you or cron has done a 'yum update' and, instead of installing a new kernel, you see the error:

At least [number]MB needed on the /boot filesystem.

This stops the update and, until it's fixed, you can't do any further updates. So, what's the fix? It's very easy really. First of all, you need to establish what kernel you are currently using. Do this like so:

uname -r

Make a note of that and ensure you understand not to remove it.

Next, we need a list of the kernels in /boot. Do this like so:

rpm -q kernel

This will give you a list of all the kernels currently residing in the /boot partition (all the ones listed by GRUB as a fallback position should the kernel upgrade cause any problems). We now need to remove the oldest kernel in this list. Say we were on CentOS and the oldest kernel on /boot was version 2.6.32-220.el6.x86_64, we would type:

yum remove kernel-2.6.32-220.el6.x86_64

If you're feeling particularly confident that all but the current kernel should be removed, do so. It is recommended to have at least two kernels on /boot, lest one cause problems. Remove the kernels, leaving the current alone then:

yum update

You should now be able to update without issue.