diff is a utility that compares text files: you give it two text files and it will tell you the differences between these two. Up to now i didn't know that the GNU-version can compare directories too but obviously it can. I have learned something new today.

Two understand how diff works let us suppose for the moment it works on lines only (it doesn't). In principle there are three possibilities:

1) a line is present in both files
2) a line is present in file 1 (only) but not in file 2
3) a line is present in file 2 (only) but not in file 1

This is the situation you have here. Your output means the two directories will contain the same files once you:

1) copy Briggs_Stratton_Generator.zip from /media/andy/MAXTOR_SDB1/Linux_Files to /media/andy/MAXTOR_SDB1/Ubuntu_Mate_18.04
2) copy Brinkmann_8109415-W.zip from /media/andy/MAXTOR_SDB1/Ubuntu_Mate_18.04 to /media/andy/MAXTOR_SDB1/Linux_Files
3) copy Brother_2240_Drivers.zip also from /media/andy/MAXTOR_SDB1/Linux_Files to /media/andy/MAXTOR_SDB1/Ubuntu_Mate_18.04

Notice, though, that two files which have the same name (all others not mentioned in the output) do not necessarily be the same: they still could differ in content, so you would have to compare file sizes too as a first step and even if sizes are the same it might be that the content is different. You would have to use diff again (this time on the two individual files) to find out.

I hope this helps.

bakunin