here is my thought
since your files total about 400 gigs that fits with my original idea above of the two WD 2TB in raid 1
but I would put everything on the 2TB raid 1 ? 400 gigs on a working 2TB is fine once it climbs to a TB or just over then we can look again at what to do
why ? again if a HD dies you keep working and have no down time while working so you can finish what you need then get it replaced ASAP but at least its a bit less a shock if you are doing stuff to loose time
also you mention not doing this raid 1 before ? no problem its super easy to do
I do want to say
raid is not back up !!! it never has been it never will be and that means any form of it !!!!!
it can help protect you from a disc failure and thats about it really as far as data security you always need a back up !!!
I do think a solid raid card like Areca are good to get but I think you can cross that bridge later
the idea of the two other 1 TB drives as I mentioned with the partitions would again give you a nice dedicated scratch that will help when needed but does not take up any space that can be used for other stuff
and the other partition will give you that drive clone should something happen to your boot ? hope not but better to have insurance than wish you did !
the bottom half of the scratch drive I would set as a normal backup of your data remember 900 gigs your data fits fine on that
and your partition below your extra boot I would set up as time machine
the reason I would do this is you control the time that you do a nightly backup of your main working files and it will never get in the way of your scratch ! time machine might kick in when you are working ? it wont really matter but why let it happen
your clone you control when it backs up ? I would do it nightly also and this way your time machine drive is free to work as it wants
also if something happens to a HD in your system you should be able to keep working and finish your job
if a 2 TB drive dies you keep working ?
if your scratch drive dies ? no biggy reassign your scratch and you still have time machine
if your time machine clone drive dies ? again no problem you still have access to your BU files should you need them
your 1TB external I would then hook up weekly and do a BU of working files and put it away some where safe ?
this 3rd layer is handy to have off line and off the computer
I would say do it nightly but really who is going to do that ? do something you know you will actually do !!!!! once a week or after a huge job ? again this is like flood insurance on your home to back up your regular insurance
so once your data grows past the point its not really fitting on your system ? I would say start archiving stuff onto externals ?
you could get a nice PM type case and put HDs in so you can access old client stuff when you want or even put them on FW drives ?
depending on the need of your old files ? if they are truly scrap after you are done thats your call to know what you can delete or not worry about
we archive stuff using our OLD hds as we update to new HDD our old stuff is copied to the new HDD and the old stuff sits where it is and gets put away !