When I travel I always carry two external hard drives. With the Mac computer you can have a complete copy of your hard drive on an external hard drive.
Making of mirror backup
SuperDuper software to create a duplicate hard drive of my computer.
There are a few benefits to having a copy of your hard drive. First of all it is a backup. If your computer hard drive fails and is not recoverable then you have everything on your backup hard drive. Well you will have almost everything, from the time you backed it up till the time you use it all that material will be lost.
There are a few options, but I pick the full backup. The first time it backs it all up and later it will only add the changes.
For the Mac user when you have an external hard drive that has everything on it, you can borrow another person's Mac and launch your computer.
Turn the computer off and then when you launch the computer just hold down the option key until you see your hard drive pop up as an option. Just click on your hard drive and now you are on your computer. Now it is not quite as fast as if it were built in, but it works great.
You see your computer cannot fix something that is running. The only way to fix it is to run it from another hard drive sometimes.
Now one more software I recommend buying is DiskWarrior. This software will do major repairs to your hard drive for you. However you must either use their CD or an external hard drive to run it. It really improves the performance of my computer.
I recommend running it about once a month.
The reason I use an external hard drive to backup my system is to have a duplicate with me in case of emergency and to help me improve the performance of my main hard drive.
Hard Drive for Pictures and Video
My second external hard drive is where I put all the photos and video I shoot on jobs. I never put them on the main hard drive.
First of all shooting with a Nikon D4 in RAW mode creates a 24 mb file. My first hard drive was only 40 mb. It will not take long to fill a hard drive shooting in RAW mode. Videos are easily hundreds of megs in size as a bare minimum. Some run a few gigs and having this size file will wear out your primary drive if you put them on and off repeatedly.
I have more than one backup of my primary drive. I have at least two more of these back in my home office. I also have multiple backups of the photos on different hard drive.
My recommendation is to always carry at least two external hard drives to do at least at a bare minimum what I am doing.