I was thinking about this today:
I’m starting to collect a bunch of 2.5" drives as I move into the realm of laptops. While I could probably find some bracket to make the 2.5" drive work inside my current Drobo, something like a ‘Mini Drobo’ would be a cool idea.
Same parts / logic, just shrunk down to 2.5" drive size.
well since synology do one:
http://www.synology.com/enu/products/DS409slim/index.php
and qnap do two (including an 8-bay one!):
http://www.qnap.com/pro_detail_feature.asp?p_id=120
http://www.qnap.com/pro_detail_feature.asp?p_id=124
i would be amazed if DRI havent at least considered one![hr]
of course i suspect that the main reason is that DRI have done surveys of their customers and most of them have around 4tb of data (i think from memory) so a 4 bay 2.5" model wouldnt hold enough (using current drives)
Yeah using 2.5 inch drives you can stuff more in the same sized chassis. An 8-slot “Mini Drobo” could hold 4TB. I’m not sold on 2.5TB drives. Because they don’t have the capacity you need to use twice as many and thus increase the risk of a drive failure. But that’s me. I’m a storage geek.
an 8 slot mini drobo could hold 5,250GB 
(at the moment!)
Ooooooo I missed that the 2.5s are up to 750GB. My soon to be delivered MacBook Pro will have a 500GB drive though, if I had a choice, I’d opt for the 256GB flash drive and just keep carting around my external FW if I need more.
They’re up to 1tb but those are 12mm thick and I’m not sure if the trays in those NAS are only for the more standard 9.5mm