How to copy a directory from one hard drive to another with every single file?

I would use rsync for this so that if there is an error (or you need to stop copying) partway through you can easily resume later, without having to recopy everything.

rsync -av /media/sdc1/Pictures/ /media/sdb1/Pictures/

cp -r /media/sdc1/Pictures/* /media/sdb1/Pictures/some_dir

• The -r is recursive, ^{read the man page...}
• With /media/sdc1/Pictures/* the asterisk is to copy all the contents of /media/sdc1/Pictures/, but not the parent directory itself.
• The some_dir of /media/sdb1/Pictures/some_dir is where you want to put it.