Very important & often ignored topic. I follow the route as discussed below :-
1. My primary working setup is an old Vaio laptop fit with a screen. It has limited capacity of 500 GB internal. So I have attached 2 X 4 TB Western Digital my book external drives. Coming back from a shoot I first dump everything into this set-up in two broad categories of "Professional" and "Personal" and keep the two eHDDs mirrored. Then I delete the obvious bad ones. Keep the obviously good ones and mark the "may bes" , all through lightroom (suitably tagged). Till the culling is complete I don't delete the camera memory cards.
2. I have another very old PC which is used offline and have been retrofitted with 2X5 TB of internal HDD and the same holds the same folders as the Vaio. This is used ONLY as a storage PC . I am planning to buy a more recent PC / NAS which can be wirelessly synced with the VAIO ( I am hopelessly ignorant of such things

).
3. I keep a third copy in another offline PC at my office ( separate physical location).
4. All the three units are manually synched every weekend by me ( I am planning to do it automatically through software in future) . In fact the two eHHDs are copied and updated by a software called "second copy".
5. I keep the final keepers in cloud and I also Cut DVDs ( planning to migrate to Blue-Ray discs soon).
This method is the outcome of my belief "two is one and one is none". Currently I am achieving redundancy, physical separation as well as locational separation and a backup copy in the web. But all at the expense of a lot of manual baby-sitting. I am aiming at higher degree of automation in a month or two.
cheers
Somnath