DSM-G600, DNS-3xx and NSA-220 Hack Forum

I think that I would try:

rsync -av /mnt/HD_a2/ /mnt/HD_b2

The rsync man page says:

A trailing slash on the source changes this behavior to avoid creating an additional directory level at the destination.
You can think of a trailing / on a source as meaning "copy the contents of this directory" as opposed to
"copy the directory by name", but in both cases the attributes of the containing directory
are transferred to the containing directory on the destination.   In other words, each of the following commands
copies the files in the same way, including their setting of the attributes of /dest/foo:

    rsync -av /src/foo /dest
    rsync -av /src/foo/ /dest/foo

So,
rsync -av /mnt/HD_a2 /mnt/HD_b2
rsync -av /mnt/HD_a2/ /mnt/HD_b2
rsync -av /mnt/HD_a2/* /mnt/HD_b2

all do different things.  The last of the three uses '*', which just matches the non-hidden files.

Since your backup disk is bigger than your source disk, you might consider using rsnapshot, which uses rsync under the covers to do pretty much what you are doing, plus it can keep "time-machine" style backups of older versions of your files.  The backup files will take more space than the source files, depending on how often they change, etc.  I use rsnapshot to shadow my A disk to my B disk; it is worth looking at.

This is probably what you want.

rsync -av /mnt/HD_a2/.  /mnt/HD_b2/.