>-rw-r--r-- 1 root root 446 Nov 28 19:23 /dev/sdc
Not sure how it got there (tho date is similar to others and
446 is the size of boot.img without partition table in a MBR)
I wonder if it's related to the sdc{1,2,3} in code in post #3:
>udevd[3094]: inotify_add_watch(7, /dev/sdc1, 10) failed: No such file or directory
(I have no idea how udev does all its 'magic')
Let us know... Best wishes...

Before any `dd ... of=/dev/sdX`, do a `ls -l /dev/sdX` to *ensure* it IS a 'block special device' ['existing']

I reproduced this strange/'impossible'/... behavior by: (on VL vbox w/ just sda & sr0)
put Ubuntu 'hybrid' netinst [50MB] mini.iso into cd;
dd if=/dev/sr0 of=/dev/sdc bs=1M (I could have used sdb, sdd, sde, sdZ, sdZZZZZ, /dev/any-non-existant)
cfdisk -P t /dev/sdc
[looks like a real device, BUT it is a *plain datafile*, which could have been in /tmp, /home, ...]

I also mv /dev/sdc /tmp and `fdisk -l /tmp/sdc` looks like a real disk!

You might watch `df /dev` as you *accidentally* made/make a FrankenDev with dd
(also watch for any '/dev filesystem FULL' messages, when FrankenDev is around)

Back in the 1990s, before udev, when /dev was just a dir in /,
backups to *non-existant* st0 tape device
used to fill up /