I use Mozy. It is $4.95 a month for unlimited storage space. I chose this one because it was one of the few that let you backup external drives. I backup on a daily basis the files from my husbands and my computer to an external drive. Then every other day I let the online backup run overnight. Every few months I run into a glitch but once I engage Mozy support it is fixed in a day or two. Early on I did a few test restores of a few files and it was fine.
One of the options for the backup is for faster computer of faster backup. I leave mine on faster backup which pretty much renders the computer unusable while the backup is running. It is fine since I let it run while I am asleep and I don't have it running from my primary computer. The external drive is connected to a secondary computer we have that doesn't get used much.
Your initial backup will take forever, perhaps a week or more. I think we have about 187 GB backed up.
Hope this helps.