As long as you have an offline copy of this data that can be used to fully restore the data on your system you are fine. A proper backup is a copy of the data that can be restored if the system or network the hot data is on completely destroyed that enables it to be completely restored data wise.

In terms of RAID, that is not a backup. RAID1,5,6,10, etc. is to help keep your system online in case of a disk failure to help give you time to replace failed disks with some options giving you increased disk read or and write performance.

I would recommend you keep something offline that you backup the data on the USB HD from time to time. Something that can do versioning would be ideal so you can view the differences over time.