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Correction: This box does NOT support H.264. You need the M-4100SH for that, which also officially supports 1080P and has an HDMI. The unit reviewed here (the M-4000SA/PA (SA=SATA, PA=IDE)) supports 1080i and unofficially some 1080P content.
It only holds one drive, but through its side door it is easy to swap out drives (no hot swap).
In addition to the USB host ports, this box can stream media direct from your PC via LAN or optional wireless adapter. It can also be configured as an FTP server for easy transfer of files to and from your PC.
It will even stream SHOUTcast Internet radio stations.
The M-4000SA/PA is available already (no need to wait till May 30) as is the M-4100SH.
How do I know all this?
I work for http://www.tvixbox.com which is where you should order yours if you are in the US :).
Let me know if you have any more questions about this fantastic device.