I don't think you can adapt esata to USB because esata does not have power. You can go the other way.
There's a mod to add USB to the DM500HD but it involves removing resistors from the mainboard. That's a bit too scary for me.
There's a bug in Crossepg which means that sometimes the guide gets written to memory instead of the hard drive. The memory can fill up and disable the box. There's a fix in a sticky at the top of this sub-forum.
Sharing a drive between 2 Dreamboxes would be useful. It should be possible to watch programs recorded by the other box.