Don't fool yourself into thinking that having a single external device for backup is suffice. Trust me, have a least two backups on separate drives, or have one a cloud device store off-site, or an online one such as Carbonite. It's not a matter of if, but when your external hard drive will fail. Choose your files well, because not everything should be backed up.
I keep all my artwork in .eps format, it all gets backed up to an online site, but for the layout stuff I do in the cutting software - that I don't mind if it doesn't get backed up. I have two NAS at the home office, and off-site cloud I keep at my brother's house (because in the even of an unattended house fire, running out of the house with my backup drives is not an option - besides, the animals come first ... oh yah, and the husband). I also have a Carbonite account to grab the stuff I need in the even of something catastrophic. Carbonite chugs along slowly, so that's why I have a dedicated cloud, I'm not sharing bandwidth with everyone else trying to get files on or off the Carbonite server farm).
I personally have my stuff (artwork and layout), categorized by type (auto, food, signage, arrows, etc.) and by customer/business (customer names). So I know where my layout files are, and where I can find my artwork if I need to use if for another customer. If you do need to preserver and backup your layout, then like Slice & Dice recommended, export it a multi-application file format like .eps, .svg, or .ai
Even a bad backup setup is better than a no-backup setup.