Your samba users need write access to the directory they are writing to. Giving full access to everyone (777 pretty much means "everyone, go wild") is the easiest way to achieve that.
It's not super secure, especially if the system is shared between users, but it should be sufficient if you just quickly want to share a drive. The more correct (but admittedly much more cumbersome) way would be to add individual users (named after your samba users) and organize them in a user group, then adjust the directory permissions so only the group has access.
you will see at the left "r", "w" and "x" indicating "user", "group", "all" levels, first "d" indicates directory. You should google "unix permissions" to know more about, this it is a standard in all unix based systems.
to get full permissions across all mounted volume sub-directories (sub-folders in windows) you should run:
chmod -R 777 /mnt/sda2
-R stands for "recursive". "777" means "rwxrwxrwx", usually defaults for directories is 755 and files 644.
If you need help securing your samba, with user-password authentication let us know.
So far all users in your network will be able to read-write (also delete) without any user login, if this is for your home in a controlled enviroment and your data it is not important (music, movies, etc) you probably will not need this.