Disasters, natural or otherwise, old PC crashing etc, are the reasons IT pros follow the 3-2-1 backup rule: Make three copies, on at least two different media – like the drive on your machine and a USB drive — with at least one copy stored offsite.
If you’re backing up files to an external drive, you’re doing more than most folks.
The next step is to get a copy stashed somewhere else. You might be capable of keeping a couple of backups, one at say, Grandma’s house or in a safe deposit box, but it’s a lot easier to use a secure online storage service like Dropbox or Box. They’ll automatically sync a folder full of your files “‘in the cloud,” which is a fluffy way of saying in a data center full of servers somewhere you aren't.
My personal favorite technique is downloading and installing backup software like CrashPlan or Carbonite. I like CrashPlan because you can use it for free to back up to a local drive, another computer that you trust, say, at your parent’s house, or, for a modest fee, CrashPlan Central’s cloud storage. It takes a while for online services to back all your data up; your Internet connection can only move lets say some maybe megabytes per minute in the end. But these servcies run automatically in the background, and, unlike me, they never forget to back up your files.
Online file services make it easy to share files too, or access anywhere you can get on the Internet.
So if you have files you can’t afford to lose, do yourself a favor, and get your own 3-2-1 backup plan rolling!