The external hard drive is formatted with an NTFS file system.
The precise command I'm using is
For some reason, it seems to take only about one minute to run the command now; earlier it took two minutes every time I ran it ...!
sudo rsync -az / --exclude=/backup --exclude=/cdrom --exclude=/dev/* --exclude=/lost+found/*
--exclude=/media/* --exclude=/mnt/* --exclude=/proc/* --exclude=/sys/* --exclude=/tmp/*
I tried using the --modify-window option but it doesn't seem to make any difference, regardless of window size.
I also tried using only the r and t flags and that takes roughly half a minute, if that's of any importance.
When I time the commands, I typically get results like those below (with -rt and -az respectively)