this kind of content-search might take a while, because it searches EVERY file.

# qnap (busybox) tested/approved

su; # become root
cd /; # go to root directory

# start search in all files
find . -type f | xargs grep -s -l "STRING THAT YOU WANT"

# explanation of options taken:
# -s == remove the "grep no such file or directory" error messages littering console