To set up the space on the external HD for the back up, do you match up the space on the external HD with the advertised HD space of a Mac? For example, a 80G HD in a Mac can be about 70GB storage after being formatted. Do you still allocate 80G on your external HD (as it is recommended to set up the dedicated partition on the external HD for back up purposes)? Or, do you just go with the actual figure which is about 70 GB?