In the 7.3.1 firmware, you can now connect a USB Drive to a Time Capsule and then archive the 1TB disk itself. That is a backup of the backup. This is supposed to be fast. However, this is a one time copy so if you don’t have enough space on your backup USB drive and delete some, the Airport Utility doesn’t update the free space available. So, if you find you don’t have enough free space, then here is the process:
# Start Airport Utility, click on the Time Capsule entry and click on Manual Setup. Then click on Disks and Archive…If things are greyed out, but you can see the USB Disk, then you don’t have enough space.
# Start Finder and go to the Time Capsule and the USB disk, delete stuff, so that you have enough free space (if the Time Capsule is using 200GB, then you need at least 200GB free on your USB).
# Close Airport Utility and restart it and the thing should be not be solid color and you can archive! BTW, while this is happening, you can’t write to the Time Capsule and the LED in the front blinks amber. So you want to do this in off hours.
It will definitely take a while, but is way faster than doing this over the network with a copy.