I use Acronis, but not on a separate HDD. And I found it so confusing at first and yet it's not.
I would suggest this.
1. Install Acronis onto the PC and use it from within Windows unless you have a major problem and absolutely have to use a boot disc. Put a shortcut to the desktop so it's always handy to use.
2. Plug your HDD in and go to "computer" and make sure it's shown correctly and that you know what it's called e.g xxxx Disc F etc.
3. Click on Acronis to open it.
4. Select tasks and create a task. The wizard will walk you through it. You want to back up the whole C drive if that is where all your progs and data are. Say no to passwords, use the default options etc. Make sure you select your HDD as the location and give your backup a name such as Sept 2009 etc and select do not run automatically.
5. Exit Acronis.
6. Open Acronis again to now make the backup. Select tasks and your new task will be listed. Click on it and the backup should start.
In a couple of days say you want to "add" to that backup so you just repeat step 6 and just the changes that have occurred since the last backup will be added.
That's enough for now :)