Looks good to me.
I guess you are using a big PSU 800-1000w so I suppose that retains enough power for a few seconds for hdd emergency shutdown routines. A UPS may be a good idea to be certain of a proper shutdown if there is a power failure, but I think you would be ok with a big psu.
I like the use of 2tb hdds. I have always been wary of going over 2tb because of the amount of time it takes to repopulate the array if (when) a disk goes down. I would do it slightly differently and use 2.5" hdds for a long list of reasons including:
~ they are made to be more robust (designed for mobile use)
~ vibrate less and can tolerate stronger vibrations (being designed for mobile use (x10 in some specs))
~ quieter
~ low power so a less powerful yet high quality quiet PSU can be used
~ smaller (you can store 4 or 5 in the space for one 3.5" when you archive them)
~ the backplane is smaller, less noisy and vibrates less
~ metal backplanes are cheaper and more rigid so even less noise and vibration
~ if you drop them onto the carpet when taking them out of the archive (the cupboard!) they should still work but a 3.5" likely would be fooked (I'm assuming you follow the model of considering the NAS as being desktop level data and migrate redundant copies of data and media files to archive disks (bluray discs are useful for multiple redundant discs of data imho (yes I've been paid to do this stuff (yes things went wrong (no I didn't give a hoot as it was someone elses data (lol (no lol if its your own data)))))))
~ they are easier to handle and so less likely to drop when using as archived disks with a dock
~ you can send them around the country by mail or courier and they should be ok, I wouldn't try that with 3.5"
Yes the IO is slower but modern 2.5" are fast enough for media use as sequential reads and writes are always fast.
Also everyone don't forget to copy and backup any IDE drives. I have at least 14 of various capacities that I have to chug through before it is too late.