Qnap just announced a new addition to their line of NAS appliances: the TS-410U. This 1U system can house 4 3.5 inch hot-swappable SATA hard drives, delivering up to 8 TB of raw capacity. It also supports 2.5 inch drives.
All the normal features for a SMB storage appliance are available, including RAID levels 0, 1, 5 and 6, and the ability to do a live migration to a different RAID level. Swapping in larger disks while the system is operational is also supported, enabling online capacity upgrades in the future. The storage can be accessed via both NFS and SMB; it also brings the iSCSI support that has been added to the QNAP software just recently.
The system is powered by a 800MHz CPU, and offers three USB ports and two eSATA connectors for future expansion or backup, plus dual gigabit LAN ports. The iSCSI target supports thin provisioning, and there is also an option for replicating the contents of the NAS to a remote unit. Full details are available here; pricing is expected to be below the $800 price of the faster TS-419U.