Did you make a snapshot before the failed system uppgrade? If yes you should be able to connect the SSD to your PC, browse into /usr/share/hassio/backup and copy your snapshots to your PC for easy recovery after the base install of your rpi4.
The boot folder should be auto-mounted and visible (on a windows machine that is, otherwise /boot)
Just connect the USB drive to a computer and you will see
Or you can run from cli:
sudo ls -la /boot
which brings up (where kernel 5.5.10 is not installed:
drwxr-xr-x 3 root root 4096 Apr 2 12:12 .
drwxr-xr-x 18 root root 4096 Feb 14 2019 ..
-rw-r--r-- 1 root root 83 Dec 31 15:19 System.map-5.9.0-0.bpo.5-arm64
-rw-r--r-- 1 root root 248519 Dec 31 15:19 config-5.9.0-0.bpo.5-arm64
drwxr-xr-x 2 root root 16384 Jan 1 1970 firmware
-rw-r--r-- 1 root root 27915840 Apr 2 12:12 initrd.img-5.9.0-0.bpo.5-arm64
-rw-r--r-- 1 root root 22656880 Dec 31 15:19 vmlinuz-5.9.0-0.bpo.5-arm64
but unfortunately no boot.cfg
Running as root (su - for all permissions over the system):
locate boot.cfg
delivers an empty result.
Ok sorry so itâs not in the boot folder. Been a couple of weeks when it happened to me and had to apply this fix.
Anyway just connect the USB disk to a computer. On a Windows computer there will be only partition mounted and that is where all the files boot files are (you donât even get to see the Linux drive what you are seeing now).
boot.cfg obviously only exists on machines running vmware.
Running as superuser
locate boot.cfg
delivers an empty result.
Which means there is no boot.cfg for Debian 10 on arm64 (as I pointed this out before).
Unfortunately no up-to-date snapshot just before the system upgrade.
Iâm not so familiar with Linux/Debian, but canât believe the recommended system upgrade can make my whole home automation unusable and there is no way to recover/rollback without full reinstall. 
There is no way to change the firmware or overwrite the wrong files manually?
This is why backups exist - to protect you against unknown issues, even moreso if you arenât familiar with what you are doing.
Always make a backup before doing any updates.
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Iâve been saying a couple of times 'just connect the USB disk to a computer and you will see what I mean". The mount/boot drive is HIDDEN but if you mount it on windows computer you will only see that partition.
I hate repeating myself over and over. Iâm not running a VM. Iâm using RPI4 with debian installation according to this guide. Thatâs why I am in this topic and not in the VM topic (if there is any).
edit config.txt (sorry my bad. As I mentioned it was a couple of weeks ago when I did this and i even posted it back then).
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Thanks you very very much. You saved my day!
After recovering this way, there is something I should do to prevent re-upgrading of this configuration file?
@Koal4 Chill, no need to yell.
Itâs a difference to advise somebody to âedit boot.cfgâ but in fact you mean âedit conf.txtâ, isnât it? Your tune is quite irritating.
However, @unlikely, youâll find that file inside /boot/firmware (not hidden at all) or as @Koal4 says if you connect your ssd to your windows machine âyou will only see that partitionâ anyway.
Would you please be so kind to report back to the community whether this workaround works? May help others with the same problem.
Yes, run:
sudo apt-mark hold linux-image-arm64
This prevents updates to update the kernel for the time being.
Actually I said sorry about the mixup (seeing it was weeks ago). But I was trying to help unlikely but you kept âde-railingâ my advise and started to fill in the blanks and assume I was using a different kind of setup.
There are 2 txt files and only 1 txt file had the 5.10.x references which was also basically a hint and confirmation what I was suggesting.
When you connect the ssd to a windows machine youâll se the only fat partition RASPIFIRM auto mounted. Inside that partition youâll see few text files and a bunch of other files. Just open and edit the mentioned text file.
If only I hadnât been so intimidated and lazy, I probably would have easily guessed yesterday which file was to be edited.
Just after successful boot donât forget to apt-mark hold otherwise next reboot fails again.
Thanks again
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I have followed this guide several times with a lot of help from many in pm and I am still having issues. The issues are consistent. Sometime after the snapshot restore I lose access rights. Usually home assistant doesnât start on a reboot. I can start it usually with portainer, but not always. When I install samba sometimes I canât restart it after the install. Once I get home assistant running, after a few hours the pi4 loses network connection. It is still running but I canât connect to it through ssh. I do not see any of these issues using the rpios (not supported) and HA supervisor or with HAOS 5.4 and below.
The SSD drive is Samsung (have tried others) and either a StarTech 3.0 or 3.1 controller. The power supply is the official 5v, 3A, but have also tried the 5v 3.5 A. The firmware is the March stable version.
I started trying the supervisor and Debian due I and many others having issues with the pi freezing or losing network connections after updating to 5.4+ (I have tried 5.5 -6.x).
Any ideas on what to try next?
try my script
What does your script do differently that would fix my issue?
Did you fix your network issues? This looks very similar to my issues. I am on a wired connection and I lose it after several hours.
I have the same issue with my wired connection. How do I fix this?
Itâs the same install, heâs just combined a few steps into one script and wanting to promote it.
You may need to set USB quirks for the external drive to boot and run correctly, assuming it is a supported enclosure/drive. From what I can remember, settings quirks can help. Have a read of this page, specifically the section on Fixing (some) USB Adapter Problems Using Quirks
Iâd be interested to hear if that helps to resolve the problem.