The cloud service I use is called Cabonite. It has been around for 10 years now and is extremely stable. For every file ( photo's in my case) I copy to my hard drive another copy goes to Carbonite. It is all automatic. I pay 59.95 per year for unlimited file space ( I have close to 300Gb of photo's to date ).

The other thing I enjoy is, I can access my photo's from anywhere, from my computer or IPad. Think of it as a great Disaster Recovery System, one copy of the file on my computer and one in the cloud.

Here is my theory on backup. You can spend a lot of money on RAID systems and large volume Hard Drives to back up your photo's . Or spend a few dollars on 1 Large volume HD and a yearly subscription to an on-line backup Service.

I currently use a 2 TB external Hard Drive and a service called Carbonite. All my photo's are updated in real time to the service. In the case of a fire, theft, HD crash etc. my files are safe on Carbonite. Also I can access my photo's from the Internet.

I have no affiliation with Carbonite but have been a user for 6 years and I am happy with the service.