been in the same boat myself.
the thing that is certain to happen is a drive failure. my nerd friend says hard drives are in two states: 'failed' and 'yet to fail'. true that.
So - Backup semantics aside, this is the #1 scenario to avoid, which means well constructed Raid system. best for photo pros is Raid 6 - which is raid 5 with an extra drive. Raid 5 is fine too, but best be cautious.
I know someone will point out that this is not strictly 'back up', but as this is what WILL happen, not what might, its worth #1 attention.
as for what might happen - catastrophic flood, fire, theft etc. yes you need off site backup. and it sounds like you are doing the right thing. make a copy and store it elsewhere. rotate and replenish. if you have lots of $ go to PhotoShelter and the kind, but for big archives its not economic.
Seagate have a decent reputation, and give 5 year warranties. thats good. Look at StorageReview.com for more info, and a reliability report. Maxtor are nowhere near this, and portable external drives generally have greatly reduced warranties, including seagate, (the enclosure heat damages the drives) so I agree with the poster who said use easy swap bare drives in a quality enclosure.
I'm on Windows PC, so you can put as many drives as you want in a case, rather than what Apple thinks you want. currently I have 12 +OS etc, hot swap, Raid 6, 8Tb. I backup to Seagates. you can get the 750Gb for under $200, and the 1Tb are dropping fast...
good luck