General :: Device Not Found When Trying To Push File

I am having this error "device not found" when I try to push a file. It's not a driver problem, it's not a USB connection problem. ADB commands work until the point where I try to push the file. Then I see "device not found".

I might be doing something wrong, I'm pretty new at this stuff. Or it might be just that the root access to my device has problems.

Here's what I do:

Code:
c:adb>adb devices
List of devices attached
3556*********09 device

c:adb>adb shell
shell@android:/ $ su
su

[Code] .....

I've also tried to push "build.prop" directly from my computer but get the same error.

I must mention that - strangely (to me at least) - after mounting /system as RW, I check the permissions using ES File Explorer's Root Explorer, they are shown as RW, I try to edit "build.prop" directly and I still get "cannot save file".

When I try to change permissions of the file using Total Commander -- to -rw-rw-rw -- I manage to do this ("Operation successful") but I am still unable to edit the file or overwrite it.