I would avoid drobo as I think they use their own software to handle archiving and I don't know how straightforward it would be to get at your files once the drives were outside of the drobo unit.
I've had good enough luck with the cheaper Meadiasonic multiple bay HDD enclosures. They come in simple eSATA/usb 3.0 port expansion
flavors. No firewire support, but it should be simple enough to add eSATA, or at least USB 3.0 to the Mac (though eSATA is really the way to go).
As a matter of interest, why are you using RAID? RAID is generally used to protect drives against failure that are in ongoing use in a system. If you are simply archiving these drives, you could always just copy over your files in duplicate and actually have 2 copies of your archived data. Simpler and safer in my opinion. With RAID, if something happens to one of the drives you file away, then you may be at the mercy of the RAID controller to rebuild you archive...