I'm running eOS Juno 6.1 on my laptop, and recently received a notification that my boot drive was full. My current kernel version is linux-image-5.13.0-35-generic. When purging out the old unused kernel versions to free up space, the linux-image-unsigned-5.11.0-43-generic & linux-image-unsigned-5.11.0-46-generic kernel versions fail to remove due to what appears to be "Directory not empty" errors.
Here is the output for the failure to remove one of these old kernels:
Also, I've previously purged the linux-image-5.13.0-27-generic kernel, but it continues to remain in the boot directory once I update grub. I'm also not certain as to why I have unsigned kernel versions within my boot directory. I've haven't installed any kernel versions manually. I do use the apt update and apt upgrade commands regularly.