2021-10-11  MODULES=DEP; A REMEDY FOR UPDATE-INITRAMFS' "NO
                         SPACE LEFT ON DEVICE"

The fix? Add MODULES=dep to /etc/initramfs-tools/conf.d/modules,
followed up by update-initramfs.

My most recent Debian install created a /boot partition of 237 M.

Starting with Debian's linux-image-5.10.0-4-amd, somewhere in March this
year, update-initramfs began reporting problems installing a new kernel
next to the current one.

Errors look like this:

,----
|  Setting up grub-efi-amd64 (2.04-16) ...
|  Installing for x86_64-efi platform.
|  grub-install: error: cannot copy `/usr/share/locale/ast/LC_MESSAGES/grub.mo' to
| | /boot/grub/locale/ast.mo': No space left on device.
|  Failed: grub-install --target=x86_64-efi  
|  WARNING: Bootloader is not properly installed, system may not be bootable
`----

or this

,----
|  processing triggers for initramfs-tools (0.139) ...
|  update-initramfs: Generating /boot/initrd.img-5.10.0-4-amd64
|  cat: write error: No space left on device
|  update-initramfs: failed for /boot/initrd.img-5.10.0-4-amd64 with 1.
|  ESC[1mdpkg:ESC[0m error processing package initramfs-tools (--configure):
|  installed initramfs-tools package post-installation script subprocess returned error exit status 1
|  Errors were encountered while processing:
|  initramfs-tools
`----

or this:

,----
|  update-initramfs: Generating /boot/initrd.img-5.14.0-1-amd64
|  
|  gzip: stdout: No space left on device
|  E: mkinitramfs failure gzip 1
|  update-initramfs: failed for /boot/initrd.img-5.14.0-1-amd64 with 1.
|  run-parts: /etc/kernel/postinst.d/initramfs-tools exited with return code 1
`----

The fix - conveniently mentioned at very top of this post - I found
(with encouragement of the Dutch Debian mailing list) in [Debian Bug
report logs - #929424: initramfs-tools: update-initramfs should not
store temporary files on /boot], which I found after reading [Debian Bug
report logs - #972396: initramfs-tools: Installation fails (no space
left on device)].


As root, create a file entitled modules in
/etc/initramfs-tools/conf.d/modules with just this one line:

,----
|  MODULES=dep
`----

And then redo, as root,


,----
|  update-initramfs -u
`----

or

,----
|  update-initramfs -k all -u
`----


The result (with currently just a single kernel installed):

================================================================
           Filesystem      Size  Used  Avail  Use%  Mounted  on 
----------------------------------------------------------------
 *before*  /dev/nvme0n1p2  237M  101M  124M    45%  /boot       
 *after*   /dev/nvme0n1p2  237M  42M   183M    19%  /boot
 ----------------------------------------------------------------

And that should leave you with enough space for a second kernel.

Debian Bug report logs - #929424: initramfs-tools: update-initramfs
should not store temporary files on /boot [1]:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=929424

Debian Bug report logs - #972396: initramfs-tools: Installation fails
(no space left on device)[2]:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=972396