The NITTY GRITTY
I don't think it's really relevant to the limited question at hand - what explains the extra 4+Gbytes on the 'rsync' backup HDD that should be a perfect mirror of the source? - but I'll post the shell script here that I've adapted very slightly from a script at http://nicolasgallagher.com/mac-osx-bootable-backup-drive-with-rsync/
There's also some other useful, relevant information there about using 'rsync' for backup.
The single line that calls '/usr/local/bin/rsync' to do the backup is this one:
sudo $RSYNC -vaxE -S --exclude-from=/Users/whoknows/rsync_excludes.txt "$SRC" "$DST"
(See the full script below for a definition of the variables and rsync flags that are used here).
What I've adapted is a basic script that backs up an entire source filesystem ($SRC), in my case the / (root) file system of my Mini, onto a target backup device ($DST), in my case an old internal HDD in the Dual G5 Tower (that I only boot up these days in Firewire target mode to use as a backup device ;-).
As it I found it, the script was not bad. It has some basic error-handling - the two if...fi conditionals - which is useful if there are access permissions problems with the source or target drive (you need to know that 'logger' puts its messages in the system log, which you can view with the CLI command 'syslog'). And the script is largely self-documenting through the comment lines introduced by # . You need to know also that a construct like if [ ! -r "$SRC" ]; then, is read "if NOT readable source, then", and you can read the whole script with no other prior knowledge (I think).
You'll need the CLI compiler tools, by the way, and a basic working acquaintance with the Terminal to roll your own 'rsync' from the sources, although you can just cut & paste following the compile recipe linked to in the first post to enter the necessary CLI command lines.
You'll find the necessary compiler tools as a 150M download at https://developer.apple.com/downloads/index.action#, if you're ever looking for them - registration is necessary, I think, but free and simple. OR, your Applestore ID should work just fine, too ;-).
#!/bin/sh
PROG=$0
RSYNC="/usr/local/bin/rsync"
SRC="/"
DST="/Volumes/QuadMiniBootBack/"
# rsync options
# -v increase verbosity
# -a turns on archive mode (recursive copy + retain attributes)
# -x don't cross device boundaries (ignore mounted volumes)
# -E preserve executability
# -S handle spare files efficiently
# --delete delete deletes any files that have been deleted locally
# --exclude-from reference a list of files to exclude
if [ ! -r "$SRC" ]; then
/usr/bin/logger -t $PROG "Source $SRC not readable - Cannot start the sync process"
exit;
fi
if [ ! -w "$DST" ]; then
/usr/bin/logger -t $PROG "Destination $DST not writeable - Cannot start the sync process"
exit;
fi
/usr/bin/logger -t $PROG "Starting rsync..."
sudo $RSYNC -vaxE -S --exclude-from=/Users/whoknows/rsync_excludes.txt "$SRC" "$DST"
/usr/bin/logger -t $PROG "End rsync"
# make the backup bootable
sudo bless -folder "$DST"/System/Library/CoreServices
exit 0
The rsync_excludes.txt that is referenced in the main line of the script just contains a plaintext list of things you don't want to copy over, one per line. I haven't added anything yet to the standard complement of exclusions (some of which are obviously critical):
.Spotlight-*/
.Trashes
/tmp/*
/Network/*
/cores/*
/afs/*
/automount/*
/private/tmp/*
/private/var/run/*
/private/var/spool/postfix/*
/private/var/vm/*
/Previous Systems.localized
/Volumes/*
