Backing up files to an external hard drive is an easy process. You just plug in the external hard 
drive via a USB or firewire port and copy and paste your files over. You can even go a step above and use automated software to copy your files for you on a monthly, weekly, or daily basis.
The question here is “Are external hard drive backups a reliable solution?” It is only as reliable as the life of the external hard drive. Well, all external hard drives have a hard drive inside of the plastic casing. This hard drive, like all desktop and laptop hard drives, have a shelf life to it. When the hard drive fails, and it will eventually albeit 1 year or up to 7 years, your backup also fails with it. What is the solution here then? 
It is recommended to have your files also backed up somewhere else whether on your desktop, laptop, or another external hard drive. The ideal backup solution is to have an automated backup on one external hard drive where you always keep the external hard drive running or just running when you know it is time for an automated backup. Then, have another external hard drive that you turn on monthly or quarterly to take a full backup of the primary external hard drive. This may sound redundant; however, this redundancy can save you time and money instead of being reactionary with data recovery efforts.