In general, anything useful needs to exist in two places and anything important must be in three places. Pretend your computer died one day; how much would you lose? It seems that you would be backing up your files once per month, does that give you enough protection?
All hard drives can fail. They haven't made a drive yet that is guaranteed to last forever. I personally prefer the hard drives that offer five year warranties as I feel they are put together a little better. The external drives usually do not use the SATA connector internally so if the enclosure breaks, you cannot simply just remove the drive and out it into a different enclosure.
In my opinion, a backup once per month seems inadequate. If you can, have two external drives and keep one offsite while backing up to the other one. Then swap them once a month.
Have you looked at using a backup system that is not a clone of your drives? I ask because each clone will take a while. If you used an incremental backup system then every backup after the first backup should be quicker (unless you generate 3 TB between backups).