Having just ONE SINGLE external hard disc for data and documents storage, would also including disc imaging backups onto it using something like Macrium Reflect Free Edition destroy the documents and data currently stored on the external disc? This question is for just being cautious.
As noted by , an image backup creates a single file and will not overwrite any previously established files on a drive. The biggest issue you potentially could run into is if the external drive does not have enough storage space left for your to save the image file to that drive, but even then it will not overwrite any other files...you would have to exit out of the imaging program to remove enough existing files on the drive yourself manually to create room...or pick another drive to use.
About the only other thing to note is that some people will tend to suggest that saving an image file of a boot drive to an external drive with data stored on it is not advisable. While I agree that it is not the best option, it is better to not backing up your boot drive at all. Ideally, it is best to have a dedicated backup drive where you would store any image files of the boot drive as well as then also storing backup of any data on the current external drive. And remember that the most important data to backup is usually NOT the system/OS files or program, but rather any user files (word processing documents, pictures, etc) as there are the ones that cannot be easily replaced. OS/system files and program files in theory are easy to replace (albeit potentially time consuming if having to re-install everything) assuming you have the install discs and/or install files accessible to you. The main reason for a system image is to allow you to be backup and running faster than doing a complete re-install of the OS and programs, not necessarily to backup the OS/system and program files. Your "first line" backup of OS/system and program files should always keeping track of your installer discs for the OS and programs and backing up any program install files that are downloaded from the web. A system image can then be a second line of defense as well the primary reason of being a faster way to recover.