Backing up data to external HD

Hi,

I used to back up my data using Time Machine on my laptop but what I didn't know was, after a certain amount of time, TM deletes old backups to make room for the new backup data. I ended up losing some videos from when my kids were infants.

I've since purchased a 2TB external HD and have been connecting it to my MBP and using TM to back up my data. My question is, is this the safest way to backup my data? Will the same thing that happened before happen again with the external HD?

My goal is to be able to use the external HD so that when I connect it to my MBP, it adds on the newest data and backs it up, but doesn't delete any of the old data. (I'm not sure if I'm explaining this correctly.)

Thank you for your help,

Alec