How to find out and fix the reason for the system not booting?

Question

NAS

Greetings!
What is the main problem - there is a NAS (No-name), in it, roughly speaking, a bootable flash drive is used (memory and controller are soldered on the board). At one point (after a power outage, the UPS died) it stopped loading. In general, I made a regular one out of this bootable flash drive (cut data + - on the board, soldered), connected it to the computer, it was determined. ext4 file system, Debian. I ran fsck -C -y -c -f /dev/sdb1 and got the following output:

spoiler

sudo fsck -C -y -c -f /dev/sdb1
fsck из util-linux 2.27.1
e2fsck 1.42.13 (17-May-2015)
rootfs: восстановление журнала
Checking for bad blocks (read-only test): done
rootfs: Updating bad block inode.
Проход 1: Проверка inodes, блокs, а также размеров
Pass 2: Checking каталог structure
Pass 3: Checking каталог connectivity
Pass 4: Checking reference counts
Pass 5: Checking группа summary information
Free блокs count wrong (257860, counted=257858).
Исправить? yes
rootfs: ***** FILE SYSTEM WAS MODIFIED *****
rootfs: 190103/950272 files (1.7% non-contiguous), 3530686/3788544 blocks

I restored the tracks, the download went a little further (judging by how it loaded when it was working and when it just died). I did dd to a new "flash drive", but the download does not go any further.
According to the indicator of the flash drive, I see that some communication is going on for the first minutes of loading, but then everything falls silent.
So - how to identify the reason why there is no further download? What to check/watch?
Well, I would see what nas gives out when loading on the screen - it would be easier, otherwise - a black box =(
PS Installing a clean system is not an option - there are no assemblies under nas, as well as the name of nas itself.

Answer 1

Blackbox is awesome. Love them.
Blind experiments, for starters
I'll think about it, I'll add it. Try it too and let me know how it turned out.
The screen is unrealistic to "solder"? What is the iron? Photo?

Answer 2

Ran fsck -C -y -c -f /dev/sdb1

Oh-ho-hoh, this seems to be some kind of curse with checkdisks :) :(
Well, even ordinary users agree and do not stop the "Check Disk" from Windows, but you, the person who managed to find and unsolder the flash drive built into the NAS , then unsolder it so that it is readable, and there too
?
you also decided to gasp at the boot part of the write operation.
In a good way, it was necessary, having received read access, to make a complete image, then it would be nice to make it as far as possible, "in a different way" (at least on a different computer) and compare the two images byte by byte or calculate the checksum.
It would also be good to run a read test to visually assess the state of the drive, and only then think about fsck.
Now it seems reasonable options to still try to identify the NAS and find the owners of the same ones in order to try to get the software through them.
Alternatively, you can also try to build an array of disks programmatically and look at the available partitions. If, in addition to your data section, you see service ones, then, depending on what is there, you can try to navigate using them.