Why does ECHO command print some extra trailing space into the file?

Question

I am reading multiple lines from a data file and append it to another file. The issue is that some extra space is in output file at end of appended lines. It is mandatory to

Answer 1

This should do the needful, including the need to comment line 100.

One: Don't use delayed expansion when it isn't necessary.
Two: Remove the trailing space in your echo command.

@(
    SETLOCAL
    ECHO OFF
    SET "_File=C:\Users\184863\Desktop\mydata.txt"
    SET "_Count="
    SET "_Comment=Your Special Comment "
)

FOR /F "tokens=* delims=" %%c IN (%_File%) DO (
    SET /A "Count+=1">NUL
    CALL ECHO.[%COUNT%] | FIND "[100]">NUL && (
        ECHO:%_Comment%%~c
    ) || (
        ECHO.%%c
    )
)>>"%~dp0temp_data.txt"

REM Replace the original file with the commented File
MOVE /Y "%~dp0temp_data.txt" "%_File%"

(
    ENDLOCAL
    EXIT /B 0
)