Rename directory variables for consistency.
Change the copy to extHDD to include the correct subdirs.
Use mkdir -p regardless; no need to test if directory exists.
Move the mkdir -p statements to just after the variables.
Make the mkdir -p statements a bit cleaner by getting out of the variables' hair.
Add mkdir -p statements for extHDD directories, just in case, so we have somewhere for the files to land.
Clarify comments on the df grep for drive mounts.