I have a Seagate Blackarmor NAS Server.
http://www.seagate.com/external-hard-drives/network-storage/business/blackarmor-nas-220/
I originally purchased this before I received my MBA for Christmas. Was using it for Acronis Backups for my home Win 7 gaming rig. But it works fine with the MBA. Supports OS X and Time Machine. Also supports DLNA and iTunes Media Server.
I still have the original 2 x 2 Tb drives in my NAS and have them mirrored for data protection instead of striped for speed. But there are guides out there on how to replace the OEM drives with larger after market drives. But I have not filled it up yet, so I have not bothered to upgrade it.