Hey Dave,
Yea, I need to add/update my photo storage article, but really you should start moving away from the Drobo's. 8TB external drives were on sale for $150 over the holiday break. As for bare enclosures for the drives, OWC really has me with their Thunderbay or Mercury enclosures. Unless you're doing massive amounts of copies or using SSD drives the performance difference between USB3 and TB2 is minimal.
Hot-swap is only for situations where you can't take a reboot - as in a server that's running 24x7 with business critical 'ish. If you can handle the reboot, or eject/swap disk, it's MUCH cheaper to go that route.
For all intensive purposes, if you're buying disks smaller than 4tb they need to be 2.5" in size. If you're buying 5tb or more is the only time to think 3.5" drives. You've got plenty of SSD space, but for what reason? So you have a 4tb OWC Mini, 2 8tb Drobo's for your two copies of BOXED. Toss in the 2x 2tb SSD's and you're carrying a lot of files.
Tell me a bit more about your normal workspace. Since you travel how do you handle backup on the road? How full are these drives, or is there lots of head room? What's your home internet connection like?
I MIGHT consider a small NAS, mostly due to your travel schedule. It would allow full access to everything remotely, though slowly via VPN, even allow for uploads from the road. I would also consider using it as a point that would have an external usb3 drive or two connected.
-Joe