If you are going to backup your entire hard drive you can either image it or clone it. When Macrium Reflect - https://www.macrium.com/reflectfree images a drive it makes one large file on your backup drive. The downside here it's not easy to extrack individual files from the backup image without installing the software in question.
Cloning on the other hand preserves the file structure on the backup from the original disk. I use both on occasion.
For my cloning, I'll be using one of my external backup drives a Verbatim Store 'n' Go that I bought back in Aug 2013.
First I open Macrium. I already have my backup drive plugged in. My main drive is listed as MBR Disk 1 and my backup is MBR Disk 2. Between the two listed drives in blue letters is Clone this disk... (1)
Just click on that. Then the clone window opens. Click on Select a disk to clone to... (2), that is your backup drive. A drop down menu will appear (3). Click on that.
Now in the clone window you'll have both your source drive and your backup drive listed. Before going on, this is just my choice, I like to delete the partitions on the Destination drive (they are going to be overwritten anyway). So I click on the Delete Existing partition option. I have to do it three times to get rid of the three partitions listed. I have three partitions on this backup drive because it's an old backup of my existing drive.
Then click on Next. Then you get a summary window, just click Finish. Then you get a warning message about the drive to be overwritten. Click continue to confirm. Then wait. You can continue using your computer for other purposes at this point but I prefer not to. I keep the windows open and wait till it finishes.
You will eventually get a Clone Completed message. In my case it took 1 hour and 35 minutes 57 seconds. This is the time from my Windows 7(64bit) desktop, 4GB RAM, with a 500GB hard drive with 84GB of used space. Depending on your setup, your times may vary.
So if your main drive gives out all you have to do is swap out your old drive for your cloned one, boot it up and life goes on.