What are my secondary backup options? So we have a setup like this: 


His MacBook Pro with 1TB, about 30% used.
Her MacBook Air with 256GB, about 30% used.
USB2 drive with 2 x 2TB configured as RAID-1 ('mirrored for safety'), about 80% used, contains 300GB family photos, 700GB family videos, 400GB music, 200GB document archives, etc. This is most often connected to (1) when archiving or doing family event video editing.
His and hers iPhones, we infrequently back them up to (1) and (2).
TimeCapsule 3GB, currently backs up (1) and (2) but not (3)


All seemed good.

Then, one day, the USB drive wouldn't start up.

After cold sweats of panic, we opened the USB drive. Luckily, this is a model where you can pop open the top and pull out a drive. One of the drives inside has died. Due to the RAID-1 configuration, the other drive worked just fine, so all our data is safe.

That was too close - losing all the family photos, videos, scans of important documents, etc, would be catastrophic for us. 

We accept that with normal usage; the drive(s) inside the USB enclosure were going to wear out at some point, so this isn't a problem with the product - although I'd think twice about buying a passively-cooled device again as manipulating gigabytes of video files is going to keep it warm for some time and heat causes equipment like this to fail quicker.

So we'll likely replace the USB drive with something suitable in the next week o so.

...but we're now really keen to incorporate some kind of secondary backup so that we don't have this panic again.

We've looked at 'cloud drive' types of solution, but they seem to presume that the entire backup will fit onto one computer - which it won't. We also don't need all our photos, videos, etc, to be fully sync'd - just be able to retrieve it if everything goes south with the USB drive.

So, putting aside costs for the moment, what are our secondary backup options?



UPDATE

I should've been clearer - the USB RAID drive is only sometimes connected. Backing up ~2TB to the cloud will take a very long time (2TB @ 2mb/second ADSL upload = 102 days according to http://www.databarracks.com/support/bandwidth-calculator/) which would tie up his or hers laptop for that whole time.

Can we connect the USB drive to the TimeCapsule and it upload to a cloud backup provider?
More than a 'cloud drive' solution, you should be looking for a 'cloud backup' solution. Backblaze and Crashplan are popular options, but there are a variety of possibilities with different features , such as Arq or Tarsnap.

A bootable copy of your main drives would be an additional insurance against disaster; you can use a software like SuperDuper! or Carbon Copy Cloner.