There are different ways of formatting X-Box drives. Sometimes the PC-based tools have trouble reading them.
That said, FTP over a 100mbps network should be fairly fast - the maximum transfer rate is about 12mbytes per second (that being 720mbytes per minute, 42gb per hour, or just under a full terabyte per day). Average rates might drop a bit if you're dealing with lots of small files, but you get the point - should only take you half a dozen hours or so to get everything done.
Remember, if you're using multiple networking devices (eg, you've got a switch and a router and maybe some other things hooked up between the console and the PC), then the maximum transfer speed will be limited to whatever the slowest device in the chain can operate at.
That said, though, your network is only transferring about 5mbytes per minute, which is waaaay under what even a humble 10mbps network should be able to handle. Try enabling/disabling "passive mode" in your FTP client. Try using a different dashboard as the FTP server on your console. If you're using other network-intensive services in the house (eg BitTorrent), try closing the clients.